2018-01-09 Bootstrap Principle In Audio Amps

FB group Building transistor radios 2018-01-09

Willie Barnett
That is one deficiency of your circuit: the output transistors do not have the ‘bootstrapped’ bias resistor.

I’ll explain why it’s needed. In your circuit, the signal turns the driver transistor as close to off as possible. The current through the base bias resistor causes the top output transistor to turn on. But the transistor’s base and emitter voltages go higher, and as those voltages approach the + supply V, the base bias resistor has less and less voltage, so it can’t supply the needed current. The transistor becomes current starved. So in your circuit the upper output transistor can’t drive the speaker close to the + supply V.

To solve this problem the spkr is connected to positive and as the voltage across the spkr goes positive it adds to the + supply V. The base bias resistor gets its current from the capacitor end of the spkr, and the bias resistor always has voltage across it, so there is no problem with current starvation.

Another way to solve this problem is to split the base bias resistor and connect the split point through a capacitor to the output. In this case the spkr can remain connected to ground.

The 4.7k can be lowered or made zero. But the circuit should have a small capacitor from collector to base of the driver transistor to prevent it from oscillating. But then maybe you might want it to oscillate, so it can drive a 50 ohm antenna at a ham frequency??

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