Beginners 250mW Amplifier
by Harry Lythall - SM0VPO


A few years ago I produced a kit for an audio amplifier. Most of the comments I received were that it is was too large, heavy, and expensive. It seems that a small little easy-to-build amplifier is really needed and appreciated. Several requests all demanded that the unit:

Ok, there is a need for such a simple circuit, so I put myself to work on the workbench for a much needed break, and designed this beginners 250mW amplifier. It will deliver up to 500mW at 12v, but it draws just 3mA quiescent current at 9.1v, and delivers a very healthy 250mW into a 15-Ohm speaker.

Circuit Description

The circuit is a fairly standard DC-coupled complementary amplifier, and is suitable for amplifying the output of the TDA7000 FM receiver. There is no rocket science used, and all components are somewhat standard, but generous negative feedback makes several different transistor alternatives possible.

TR1 is biased to 33% of the supply voltage at the base terminal. This transistor is a PNP device, and the collector drives TR2 directly. TR2 also amplifies, and has a collector load of a 4700-Ohms (4K7) resistor to the battery supply, via the speaker. The speaker is (almost) a short-circuit when compared to the 4K7 resistor.

D1 and D2 are inserted in series with TR2 collector load so that there is a constant 1.3-volt drop. The ends of the diode "dropper" set the operating conditions of TR3 and TR4. These two transistors are emitter-followers, but one is a NPN device (TR3) that will drag the load more positive, and the PNP device (TR4) applies current to drag the load in a negative direction - but not at the same time.

TR2 collector load is routed through the speaker. This means the signal voltage will be added to the supply voltage and give a more linear output from the amplifier. This technique is called "bootstrapping". the speaker must have an impedance of 15-Ohms. The amplifier will drive an 8-Ohm speaker, but with reduced power.

TR1 and TR2 each invert the phase of the input signal, so the output is in-phase with the input signal. Feedback from the emitters of TR3 and TR4 are fed back to TR2 emitter, so TR1 will not invert the signal. This input to the amplifier causes the output to be in the opposite phase - in effect, cancels everything. TR1 therefore compares the output voltage from TR3/TR4 emitters with it's 35% bias. Any error is fed back to form a correction and maintain the output DC bias level at 50% of the supply voltage.

The feedback, however, is routed through a 2200-Ohm resistor (2K2), and the addition of a capacitor and resistor to ground means that the audio feedback is reduced: divided: attenuated. In order to get a total cancellation, the output signal must be larger than the input signal by an amount equal to the voltage divide rate of the resistors, plus 1.

The capacitor has no long-term DC effect, so the DC feedback is not affected. In effect, the gain of the amplifier can be set by selecting R4 and R5, according to the following formula:

Voltage gain =R5+ 1

In effect, you can select the value of R4 to suit whatever purpose you want to apply the amplifier. Here is a general guide for selecting R4:

GainTypical applicationR4 value
1High-level 5v from D-A converter, or oscillatorNot fitted
32V from "Walkman" or computer LINE OUT1000-Ohms
51V from "Walkman" or computer LINE OUT470-Ohms
22250mv from high-level wireless detector100-Ohms
50100mv from low-level wireless detector47-Ohms
10050mv from microphone22-Ohms **

** Low values of R4 can cause distortion.

D3 is a 1-Ampere protection diode: a "Wally diode". This is fitted because the ease with which one can apply the battery the wrong way round. In this event absolutely nothing will happen. D3 will not conduct when the battery is the wrong way round. The price to pay is that the battery supply falls from 9V to just over 8V, which restricts the maximum output from the amplifier.


This is easy. All but one resistor are fitted horisontally. Fit them all first. Then fit the three diodes. Fit the capacitors, and finally the transistors. Miniature capacitors are needed due to the space restrictions on the board, but this will give the constructor a good bit of practice with the component order. Capacitors may be mounted a little high, if necessary.

Provision has been made on the board for a potentiometer for a volume control. If your source already has one, for example, a walkman, then you do not need to fit one and the pads on the board can be used to anchor the input wires.


Below you can see the prototype unit I built. I made the PCB and assembled it in one evening. I used an 8-Ohm speaker with series resistor (and without) to test the board and it seemed surprisingly stable. The frequency response is limited by the output capacitor you use. With a 15-Ohm speaker and a 220uF capacitor, you will start to see a fall-off in response at about 100Hz. But this is more than adequate for most audio applications, even music for the bedside radio.

If you drive 36-Ohm headphones then the low-frequency response will be quite adequate for "dunka-dunka" pop music sounds. But if your interest is in RAP music, then quality has no meaning :-).

The output level of the amplifier is about 7v peak-to-peak, or a little over 2v RMS. As you can see above (left), there is no visible crossover distortion, and clipping occurs quite dramatically if the unit is overloaded, above (right). But the novice does not need anything to setup or align the amplifier. The only real test to make is to check the DC output at the emitter of TR3 and TR4. They should both lie around 4.5v DC. If the voltage is way out then check your wiring.

Very best regards from Harry Lythall

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