Ballast for energy-saving lamps

Ballast for energy-saving lamps

This compact ballast is intended for driving a 20-watt ‘bare’ Compact Fluorescent Lamps (CFL) tube or bulb, that is, one without a driver circuit built into its socket that makes it ready to screw into an existing lamp socket. Pin-base CFLs are designed to be used with a separate ballast. As with a linear fluorescent system, the lamp and ballast must be compatible. Pin-base CFLs are available in low-power versions to replace incandescent light bulbs and in medium- and high-power versions to take over from linear fluorescent lamps or even high-intensity discharge (HID) lamps.

Making a pin-base CFL light

The circuit shown in Inductor picture uses a dedicated integrated circuit type FAN7710 from our friends at Fairchild. As illustrated in Figure 4, this device combines one high-side 625-V gate driver circuit, two 550-V MOSFETs, afrequency control circuit and a shunt regulator –– plus active ZVS control and an open lamp detection function, all crammed into one ultra-compact 8-way DIP package. Its high functionality and built-in protection features save board space, reduce power dissipation and guarantee enhanced reliability in end systems. Good!

The AC line input voltage (here, 230 VAC 50 Hz) is rectified to provide a bus voltage of approximately 320 volts DC. Startup resistor R1 supplies initial (micro-) power to the FAN7710 IC. The IC begins to oscillate and the charge pump circuit consisting of C2, D2 and D7 supplies the current to the VDD pin, which gets regulated through the internal 15-V shunt regulator.

The FAN7710’s oscillator circuitry employs three discrete frequencies: one to pre-heat the CFL gas; one to ignite it and one for the on state — see the inset for the associated (simple) maths. In addition to this, it protects the ballast circuitry from low AC as well as lamp removal conditions.


Making the inductor
The bare PCB, FAN7710N IC and the 2.5-millihenry inductor used in the circuit come as a set from the Elektor Shop. However we would not discourage anyone from purchasing the inductor parts and making it yourself.


Let’s first carefully write down the specifications:

Inductance: 2.5 mH
Core material: Epcos N19 or equivalent
Core size: 20 / 10 / 6
Bobbin: E19
Gap: 1.5 mm
Wire gauge: 0.2 mm (SWG #32)
Number of turns: 280

Now look at the construction details.
First, wind the 280 turns of enamelled copper wire (ECW) on the E19 bobbin. Bare the wire ends for about 5 mm by scratching with a scalpel, then pre-tin. Check continuity of the coil. Put the Ecore halves over the bobbin as shown, then insert and adjust the spacers to get the required air gap of 1.5 mm which is essential to achieve the required inductance. The final step is to wrap electrical isolation tape around the core frame.


Ballast for Energy-Saving Lamps Printed Circuit Board )PCB)

Elektor labs have designed a circut board for the project; the component mounting plan is shown in PCB. Thecopper track layout is available as a free .pdf file from our website at Elektor for those wishing to etch their own circuit board. Reflected and non-reflected artwork is included in the .pdf file for your convenience. Component stuffing is a breeze as only normal size leaded components are used on a spacious board. The wiring to the mains and the lamp, and all connections and connectors in between, should comply with electrical safety guidelines. (Author: T. A. Babu, Elektor Magazine, 2008)

Veroboard and How to Construct an Electronics Circuit

Electronics Circuit

In electronics, one has to know about circuit building. Circuit building is an essential skill if one is interested to build robots. Robots, need a brain or rather an intelligent brain that tells them what to do. In this case, electronic circuits are the controller for the robots.

There are the bread board, ‘vero board’ and ‘PCB’ (printed circuit board). These three are the types of electronics circuits. They are widely used all over the world. The cheapest one, is the ‘vero board’ and also one of the most popular technique.

A ‘vero board’ will have many holes lined up horizontally and vertically. What you need to know, is that each rows (horizontal) can be thought of as a horizontal wires. So, a ‘vero board’ contains many horizontal wires, row by row. If you have ten horizontal rows of holes, then you have ten horizontal wires. Note that you still need to have basic common sense of how to connect a complete, closed circuit.

With this new knowledge, now you can start building an electronics circuit. To insert the components into the board, you have to solder. Soldering is whereby you heat up the soldering iron tip until it is hot enough. Then you heat the particular spot you want to solder, for about 3 seconds maximum. Immediately apply a strand of flux material. This has to be done only once, ideally. If you have to re-solder the same part, it is no good.

However, take into account that soldering is a skill. The more you solder, the better you will be at it. Be wary of cold solder joints. A good soldering is when you see somewhat a shine on the joint. But, if you see dull looking texture, that appears greyish, then you have cold solder joint. A cold joint contains holes and cracks and voids inside. Therefore, it is not desirable and will produce joint failures in long term.

By Alfred Chai Wei Liang

WiTricity notes

WiTricity, a portmanteau for "wireless electricity", is a trademark of WiTricity corporation[1] referring to their devices and processes which use a form of wireless energy transfer including resonant energy transfer etc., the ability to provide electrical energy to remote objects without wires using oscillating magnetic fields. The term WiTricity was used for a project that took place at MIT, led by Prof. Marin Soljačić in 2007.
In April 27, 2011, car maker Toyota made an investment in WiTricity.

Technical background

Overview
WiTricity is based on strong coupling between electromagnetic resonant objects to transfer energy wirelessly between them. This differs from other methods like simple induction, microwaves, or air ionization. The system consists of transmitters and receivers that contain magnetic loop antennas critically tuned to the same frequency. Due to operating in the electromagnetic near field, the receiving devices must be no more than about a quarter wavelength from the transmitter (which is a few meters at the frequency used by the example system). In their first paper, the group also simulated GHz dielectric resonators. The WiTricity devices are coupled almost entirely with magnetic fields (the electric fields are largely confined within capacitors inside the devices), which is argued to make them safer than resonant energy transfer using electric fields (most famously in Tesla coils, whose high electric fields allow them to be used as lightning generators), since most materials couple weakly to magnetic fields (Kurs, 2007). The WiTricity devices are also claimed to be unusual in that they support efficient energy transfer for "mid-range" distances several times larger than the diameter of the resonant objects (Karalis, 2007).
Unlike the far field wireless power transmission systems based on traveling electro-magnetic waves, WiTricity employs near field resonant inductive coupling through magnetic fields similar to those found in transformers except that the primary coil and secondary winding are physically separated, and tuned to resonate to increase their magnetic coupling. These tuned magnetic fields generated by the primary coil can be arranged to interact vigorously with matched secondary windings in distant equipment but far more weakly with any surrounding objects or materials such as radio signals or biological tissue.
In particular, WiTricity is based on using 'strongly-coupled' resonances to achieve a high power-transmission efficiency. Aristeidis Karalis, referring to the team's experimental demonstration, says that "the usual non-resonant magnetic induction would be almost 1 million times less efficient in this particular systemThe researchers suggest that the exposure levels will be below the threshold for FCC safety regulations, and the radiated-power levels will also comply with the FCC radio interference regulations.
Researchers attribute the delay in developing wireless-power technology to limitations of well-known physical laws and a simple lack of need. Only recently have modern consumers obtained a high number of portable electronic devices which currently require batteries and plug-in chargers.[3]
[edit]Experimental demonstration
The MIT researchers successfully demonstrated the ability to power a 60 watt light bulb wirelessly, using two 5-turn copper coils of 60 cm (24 in) diameter, that were 2 m (7 ft) away, at roughly 45% efficiency.[6] The coils were designed to resonate together at 9.9 MHz (≈ wavelength 30 m) and were oriented along the same axis. One was connected inductively to a power source, and the other one to a bulb. The setup powered the bulb on, even when the direct line of sight was blocked using a wooden panel. Currently, researchers have been able to power a 60 watt light bulb at roughly 90% efficiency at a distance of 3 feet[citation needed].
The emerging technology was demonstrated by Eric Giler, CEO of the US firm WiTricity, at the TED Global Conference held at Oxford in July 2009.[7][8] In this demonstration, Giler shows a WiTricity power unit powering a television as well as three different cell phones, the initial problem which inspired Soljacic to get involved with the project.
[edit]Radiation levels
See also: Electromagnetic radiation and health
The company's FAQ [1] claims that it uses a "non-radiative mode of energy transfer, relying instead on the magnetic near field. Magnetic fields interact very weakly with biological organisms—people and animals—and are scientifically regarded to be safe." No actual studies or reports are claimed of the specific technology, power levels and use in home environments but it does claim that "WiTricity products are being designed to comply with applicable safety standards and regulations." No clinical field study radiation levels from in-home tests are reported on the WiTricity web site as of November 2010.

Who Invented Touch Screen Technology?

Touch Screen Technology - How It Works

There are three components used in touch screen technology:

• The touch sensor is a panel with a touch responsive surface. Systems are built based on different types of sensors: resistive (most common), surface acoustic wave, and capacitive (most smart phones). However, in general sensors have an electrical current running through them and touching the screen causes a voltage change. The voltage change signals the location of the touching.
• The controller, is the hardware that converts the voltage changes on the sensor into signals the computer or other device can receive.
• Software tells the computer, smartphone, game device, etc, what's happening on the sensor and the information coming from the controller. Who's touching what where; and allows the computer or smart phone to react accordingly.

Of course, the technology works in combination with a computer, smart phone, or other type of device.

Resistive & Capacitive Explained

According to Malik Sharrieff, an eHow Contributor, "the resistive system is comprised of five components, including the CRT (cathode ray tube) or screen base, the glass panel, the resistive coating, a separator dot, a conductive cover sheet and a durable top coating."

When a finger or stylus presses down on the top surface, the two metallic layers become connected (they touch), the surface acts as a pair of voltage dividers with connected outputs. This causes a change in the electrical current. The pressure from your finger causes conductive and resistive layers of circuitry to touch each other, changing the circuits' resistance, which registers as a touch screen event that is sent to the computer controller for processing.

apacitive touch screens use a layer of capacitive material to hold an electrical charge; touching the screen changes the amount of charge at a specific point of contact.

History of Touch Screen Technology

1960s

Historians consider the first touch screen to be a capacitive touch screen invented by E.A. Johnson at the Royal Radar Establishment, Malvern, UK, around 1965 - 1967. The inventor published a full description of touch screen technology for air traffic control in an article published in 1968.

1970s

In 1971, a "touch sensor" was developed by Doctor Sam Hurst (founder of Elographics) while he was an instructor at the University of Kentucky. This sensor called the "Elograph" was patented by The University of Kentucky Research Foundation. The "Elograph" was not transparent like modern touch screens, however, it was a significant milestone in touch screen technology. The Elograph was selected by Industrial Research as one of the 100 Most Significant New Technical Products of the Year 1973.

In 1974, the first true touch screen incorporating a transparent surface came on the scene developed by Sam Hurst and Elographics. In 1977, Elographics developed and patented a resistive touch screen technology, the most popular touch screen technology in use today.

In 1977, Siemens Corporation financed an effort by Elographics to produce the first curved glass touch sensor interface, which became the first device to have the name "touch screen" attached to it. On February 24, 1994, the company officially changed its name from Elographics to Elo TouchSystems.

• Elographics Patents
• US3662105: Electrical Sensor Of Plane Coordinates
  Inventor(s)Hurst; George S., Lexington, KY - Parks; James E., Lexington, KY
  Issued/Filed Dates:May 9, 1972 / May 21, 1970
• US3798370: Electrographic Sensor For Determining Planar Coordinates
  Inventor(s)Hurst; George S. , Oak Ridge, TN
  Issued/Filed Dates:March 19, 1974 / April 17, 1972

1980s

In 1983, the computer manufacturing company, Hewlett-Packard introduced the HP-150, a home computer with touch screen technology. The HP-150 had a built in grid of infrared beams across the front of the monitor which detected finger movements. However, the infrared sensors would collect dust and require frequent cleanings.

1990s

The nineties introduced smart phones and handhelds with touch screen technology. In 1993, Apple released the Newton PDA, equipped with handwriting recognition; and IBM released the first smart phone called Simon, which featured a calendar, note pad, and fax function, and a touch screen interface that allowed users to dial phone numbers. In 1996, Palm entered the PDA market and advanced touch screen technology with its Pilot series.

2000s

In 2002, Microsoft introduced the Windows XP Tablet edition and started its entry into touch technology. However, you could say that the increase in the popularity of touch screen smart phones defined the 2000s. In 2007, Apple introduced the king of smart phones, the iPhone, with nothing but touch screen technology

X-Ray Defender

Backscatter X-Ray Detector

Backscatter X-ray imaging devices have been in the news of late, primarily due to their use in U.S. airports and other public facilities. But, not everyone is aware that the technology is also being deployed in ordinary vans that can image the insides of passing cars and trucks, and even peer into the interior of homes and businesses. In order to penetrate the exteriors of vehicles and buildings, the x-ray beam is substantially more intense. In the wrong hands, the beam could even serve as an undetectable weapon. That is, until now. the X-Ray Defender is designed to detect the short, powerful X-ray beam from such scanners, giving the owner time to hightail it out of there, before his hair catches on fire. Remember, the X-rays will zip right through aluminum foil hats, too!

The X-ray Defender employs an ordinary PIN photodiode to detect x-rays, instead of light. The photodiode must be kept in the dark, hence the heatshrink tubing sealed with "liquid tape" cap. The uncovered diode can be seen in the insert on top of the battery. It isn't clear how powerful the scanning beam is, so it's possible that the photodiode could directly drive the SCR, without any amplification. But, it was decided to enhance the sensitivity, stretch the response to short pulses, and make the device respond only to sudden field changes. The prototype also has a small block of scintillation plastic sitting on top of the photodiode to further enhance the sensitivity, but the beam will almost certainly trigger this circuit without it. The amplifier is intentionally "starved" for current so that it doesn't respond to smaller signals, causing false triggering and the standby current is virtually zero.

Here's how it works: The powerful pencil beam of x-rays sweeps across the photodiode, causing a sudden current flow from the diode into the base of the MPSW45. The collector pulls low and current flows up through the 1N5711, clamping the gate of the SCR near zero volts. The dwell time of the pulse isn't known, but this circuit will respond rather quickly. When the x-ray beam moves away, the transistor turns off and the voltage on the collector swings up. The other side of the coupling capacitor also swings up and current flows into the gate of the SCR, triggering it. This slower waveform has plenty of width and energy to trigger a sensitive-gate SCR. The buzzer is simply reset by cycling power, a fairly easy operation to perform while running. When power is first applied, the circuit will trigger. Let it buzz for a few seconds then turn the power off and back on quickly. Try again if it continues to buzz. When the power is on and the unit is not triggered and buzzing, the power consumption is practically zero, so the unit may be left on. The tendency to trigger when power is first applied serves as a circuit/battery check.

Component notes: The PIN photodiode in the prototype is an Advanced Photonics, SD200-11-31-241 which has a 0.2" active area. Most other types should work. The high-value 62 megohm resistor provides a path to ground for any leakage and it could probably be left out for even more sensitivity, if there is no significant leakage from the photodiode. Or, a 22 megohm could be substituted. The NPN darlington isn't critical, but it should be a modern small-signal type. The 1N5711 is a small-signal schottky diode but an ordinary silicon diode will also work. The SCR should be a sensitive-gate type, but substitutions are fine. The buzzer is a low-current type designed to operate on 9 volts. The capacitor across it keeps the intermittent current it consumes from resetting the SCR.

Keep the unit nearby or build several.

Continental USA Breakfast

Class-A Audio Amplifiers

A class-A audio amplifier is pretty wasteful of power but when plenty of power is available the simplicity is attractive. Here is a simple darlington transistor example intended for use with a 5 volt power supply:

This circuit and the following aren't for beginners; they are of limited usefulness and require an understanding of the underlying principles and potential applications. They all pass DC through the speaker which is wasteful and can cause problems for the inexperienced builder. If built without variation, they should perform as described but make sure to read the text.

The 5 volts should be provided by a regulated power supply. The efficiency is below 25% and significant DC current flows in the speaker and that additional power should be figured in to the power rating of the speaker. But look how simple it is! The voltage gain is only about 20 and the input impedance is about 12k. The schematic shows two values of bias resistor to be used with the corresponding speaker impedance. With the 150k bias resistor and 8 ohm speaker, the circuit draws about 210mA (1 watt) and can deliver about 250 mW to the speaker which is plenty of volume for most small projects.   The speaker should be rated at 500 mW or more and should exhibit a DC resistance near 8 ohms (perhaps 7 ohms). Check the candidate speaker with an ohmmeter; much below 7 ohms will cause excessive current draw. With the 220k resistor and 16 ohm speaker, the circuit draws about 100 mA (500 mW) and delivers about 125 mW to the speaker. The 16 ohms speaker should be rated at 200 mW or more and exhibit nearly 16 ohms of DC resistance. (Most small speakers have a DC resistance near the rated impedance and that resistance is used to set the quiescent current level in this circuit.) Other NPN darlington transistors will work but choose one that can dissipate 1 watt minimum. Most power types don't need a heatsink but tiny TO92's might overheat.

If the inefficiency of the class-A hasn't dissuaded you yet, here is a 4-transistor amplifier suitable for small signals:

The input impedance is about 5000 ohms and the frequency response is flat from 30 Hz to over 20,000 Hz. With the 8 ohm speaker the current drain is about 215 mA and the gain is about 1700 (64 dB). With the 16 ohm speaker the current gain is about 110 mA and the gain is about 2500 (68 dB).  A volume control may be added by connecting one end of a 5k potentiometer to ground, the wiper to the amplifier input. The other end of the pot becomes the input.

Lets face it; just about any of the various IC audio amplifiers make more sense than this inefficient design. But, this circuit uses parts with only 3 legs. Umm, it doesn't use large capacitors except for the power supply bypassing. Lets see, its more fun-ariffic.  Well, lets see if we can come up with a project that takes advantage of the inefficiency:

So, what is it?

It is a modulated light sender! Connect the input to an audio source or microphone (a speaker will work) and the audio will amplitude modulate the light intensity. The inefficiency of the class-A works in our favor now, lighting the lamp to mid-brightness with no audio present. Actually, with a 4.7 volt bulb, the lamp will be near full brightness and will be "overdriven" on sound peaks. A higher voltage bulb will last longer but will be dimmer. Try a 6.8 volt bulb as a compromise. With a sensitive detector like a phototransistor, this communicator will work several hundred feet (at night). Best range is realized if the bulb is mounted in a typical flashlight reflector and the detector is similarly mounted. The input capacitor is reduced to .01 uF to give the amplifier a high-pass character to compensate for the slow response of the bulb. The audio will sound a bit muffled, anyway. The clever designer could use this amplifier for the receiver, too, switching the speaker to the input for transmitting and to the output for listening. If you choose a detector with good infrared response, like a pin photo diode, you can add plastic IR filters to block out ambient light and make the communicator harder to see at night.

Increasing the voltage to 12 VDC, replacing the bulb with a  3 watt, 16 ohm speaker and replacing the .01uF with a 1uF gives an audio amp that will deliver nearly 1 watt of audio power. The speaker will get warm, however! (Due to the nearly 2 watts of DC power in the speaker coil.)

how to make Crystal Radio (and other purpose) Audio Amplifier

Crystal Radio (and other purpose) Audio Amplifier

Here is a simple audio amplifier using a TL431 shunt regulator. The amplifier will provide room-filling volume from an ordinary crystal radio outfitted with a long-wire antenna and good ground. The circuitry is similar in complexity to a simple one-transistor radio but the performance is superior (with the exception of the amazingone-transistor reflex ). The TL431 is available in a TO-92 package and it looks like an ordinary transistor so your hobbyist friends will be impressed by the volume you are getting with only one transistor and the amplifier may be used for other projects, too. Higher impedance headphones and speakers may also be used. An earphone from an old telephone will give ear-splitting volume and great sensitivity! The 68 ohm resistor may be increased to several hundred ohms when using high impedance earphones to save battery power.

Here is the amplifier used to boost the output from a simple crystal radio. The volume control is at the bottom left and the other components are on the terminal strip at the bottom of the picture. This is a really quick and easy audio amplifier!

how to make Op-Amp Audio Amplifier

Op-Amp Audio Amplifier

The above circuit is a versatile audio amplifier employing a low cost LM358 op-amp. The differential inputs give the amplifier excellent immunity to common-mode signals which are a common cause of amplifier instability. The dotted ground connection represents the wiring in a typical project illustrating how the ground sensing input can be connected to the ground at the source of the audio instead of at the amplifier where high currents are present. If the source is a power supply referenced signal then one of the amplifier inputs is connected to the positive supply. For example, an NPN common-emitter preamplifier may be added for very high gain and by connecting the differential inputs across the collector resistor instead of from collector to ground, destabilizing feedback via the power supply is greatly reduced.

My utility amplifier was built into an aluminum Bud box and eventually ended up bolted to the bottom of a shelf as shown. The well-behaved and ready-to-go amplifier is really handy.

As is often the case, the circuit values are not critical. Other op-amps will usually work but a bit of experience may be necessary if problems develop. The two 4.7 ohm resistors in the emitters may be replaced with a single 10 ohm resistor in either position - I just like the symmetry!

how to make 4-Transistor Amplifier for Small Speaker Applications

4-Transistor Amplifier for Small Speaker Applications

The circuit above shows a 4-transistor utility amplifier suitable for a variety of projects including receivers, intercoms, microphones, telephone pick-up coils, and general audio monitoring. The amplifier has a power isolation circuit and bandwidth limiting to reduce oscillations and "motorboating". The values are not particularly critical and modest deviations from the indicated values will not significantly degrade the performance.

Three cell battery packs giving about 4.5 volts are recommended for most transformerless audio amplifiers driving small 8 ohm speakers. The battery life will be considerably longer than a 9 volt rectangular battery and the cell resistance will remain lower over the life of the battery resulting in less distortion and stability problems.

The amplifier may be modified to work with a 9 volt battery if desired by moving the output transistors' bias point. Lowering the 33k resistor connected from the second transistor's base to ground to about 10k will move the voltage on the output electrolytic capacitor to about 1/2 the supply voltage. This bias change gives more signal swing before clipping occurs and this change is not necessary if the volume is adequate.

As before, the two 4.7 ohm resistors may be replaced with a single 10 ohm resistor in series with either emitter.

how to makeComputer Audio Booster

Here is a simple amplifier for boosting the audio level from low-power sound cards or other audio sources driving small speakers like toys or small transistor radios. The circuit will deliver about 2 watts as shown.  The parts are not critical and substitutions will usually work.  The two 2.2 ohm resistors may be replaced with one 3.9 ohm resistor in either emitter.

Curiously Low Noise Amplifier

Curiously Low Noise Amplifier

The Curiously Low Noise Amplifier takes advantage of the wonderful noise characteristics of the 2SK170 JFET that boasts a noise voltage below 1 nV/root-Hz and virtually no noise current. The noise voltage of the amplifier is only 1.4 nV/root-Hz at 1 kHz, increasing to only 2.7 nV/root-Hz at 10 Hz. The noise current is difficult to measure, so this simple utility amplifier can see the noise from a 50 ohm resistor and a 100k resistor, too. (The 1.4 nV input-referred noise will increase to about 1.7 nV with a 50 ohm resistor, instead of a short, and a 100k resistor will give an input-referred noise near 40 nV, with very little contribution from the amplifier.)

This amplifier is a "utility" amplifier with a gain of 100, that would typically be used in a lab setting to boost tiny signals for measurement or further processing. It isn't intended to drive a speaker or headphones directly. (It could drive the LM386 quite nicely.) The circuit is a simple discrete transistor feedback circuit with two gain stages and a unique class-A output buffer:

• The 2sk117 is from the "BL" Idss current range and is selected for an Idss near 7 mA. The drain resistor is adjusted to achieve about 4 volts on the drain and the value depends on the Idss of the JFET.
• Most of the resistors aren't critical, but precision values are shown because the resistors should be metal film types for best noise performance. Approximate DC voltages are shown for helping with resistor selection. Deviating from the shown voltages will reduce the available output voltage swing, but the amplifier might work fine for smaller signals. Unloaded swing should be about 6 volts, p-p with about 60 mV p-p input, before distortion is observed.
• The MPSA18 acts as a noise filter. High gain is desirable here to keep the value of the base filter capacitor reasonable, but a 2N4401 could be substituted by reducing the 10k and 120k by a factor of 5. The filter will still be rolling off the noise voltage from the 15 volts supply above about 0.2 Hz. But some power supplies can be really noisy!
• The 0.1 uF capacitors serve as bypass capacitors but mainly as terminals for holding the components. These are the white rectangles seen in the photo.
• The feedback resistor is selected for a gain of exactly 100 and the value is well above the expected 1k, due to the limited open-loop gain of the simple circuit.
• A small resistor is included in series with the output for stability and that resistor can reduce the gain a bit when driving a lower resistance load. The designer may choose to set the gain for that particular load, say 75 ohms, or for a high impedance load. The circuit can drive a lower resistance than 100 ohms,  but the swing will be somewhat limited. It may be possible to leave out the 33 ohm resistor without stability issues. (Usually, such a utility amplifier is driving a much higher resistance load, typically 600 ohms or above.) Note: To give you an idea of how you can play with the output resistance, I just changed my unit's series output resistor to 55 ohms and adjusted the gain for 55 dB, when driving 75 ohm loads. Unloaded, the gain is exactly 5 dB higher at 60 dB. This way, I have even number gains whether driving a 75 ohm instrument or a high-Z device. The output buffer has no trouble driving the total 125 ohm load, with a swing limit of about 3.5 volts, p-p.
• The output stage is an unusual self-biasing arrangement where the PNP holds the gate-source voltage near 0.6 volts, running the JFET somewhat below its Idss. The 2N5486 was chosen to not waste too much current, but a higher Idss JFET will give more drive capability, if desired.

• Input Impedance: 47 megohm (set by bias resistor), shunted by 20 pF
• Output Impedance: 36 ohms, set by series resistor plus about 3 ohms from the circuit. My 55 ohm resistor mentioned above gives an output Z of about 58 ohms and exactly 5 dB of gain loss from no load to 75 ohms.
• Output voltage swing: 6 volts p-p into a high impedance load.
• Gain: 100 (40 dB) set by feedback resistor. Lower gain could be selected for wider bandwidth.
• Frequency Response: flat from below 1 Hz to above 2 MHz.
• Input Noise: 1.4 nV, rising to 2.7 nV at 10 Hz. Noise current has eluded measurement so far, but it's really low. With a 97.3 k resistor (100k in parallel with 3.6 meg) connected across the input, the noise voltage measures within a tiny fraction of a dB of 40 nV, so little to no noise current is seen. In fact, this amp and a selected resistor make an inherently accurate noise source. Connect a 152k across the input (in a shielded box), and you have a precise 5 uV/root-Hz noise source throughout the audio spectrum (50 nV times 100). A quick measurement at 40 Hz gives 770 nV/root-Hz with nothing connected; the 47 megohm is expected to contribute 867 nV. That's pretty close and still little noise current from the FET.

For even better performance, the bipolar stages could be replaced with a low noise op-amp. The input noise would drop a little, perhaps to 1 nV, as would the input capacitance, perhaps below 10 pf. Compensating the op-amp might be a bit of a challenge.

Simple LM386 Audio Amplifier

This simple amplifier shows the LM386 in a high-gain configuration (A = 200). For a maximum gain of only 20, leave out the 10 uF connected from pin 1 to pin 8. Maximum gains between 20 and 200 may be realized by adding a selected resistor in series with the same 10 uF capacitor. The 10k potentiometer will give the amplifier a variable gain from zero up to the maximum