Here I redesigned the class AB audio amplifier that I had posted many years ago. In the original design, the microphone signal had to fight against the relatively low resistance of the biasing network. This often resulted in a muffled sound, lost treble, and a signal that was weak before it even reached the amplification stage. By adding the bootstrapped emitter follower Q1, we create an 'invisible' bridge for the audio. Let's first look at the circuit diagram.
Circuit Diagram
Below is the circuit diagram.
Bootstrap Circuit
In the above schematic, the bootstrap circuit is formed by the Q1 (the emitter follower transistor) together with the bootstrap capacitor (C2) and the biasing resistors R1 and R2. The R1 and R2 provides the DC bis to the Q1 base. The BJT transistor Q2 buffers the audio signal from the microphone and the capacitor C2 connects emitter signal back the R1 and R2 junction.
When the audio signal swings, C2 feeds part of the emitter voltage back into the bias divider. This makes the voltage across R1–R2 “follow” the signal, so from the microphone’s perspective those resistors appear to have a much higher impedance.
To clarify, the microphone is a high impedance source. It produces voltage signal but little current. Without the bootstrap circuit, the microphone or the source sees the 100KOhm load formed by the biasing resistors R1 and R2. The microphone or source tries to drive that load but fails and the result is drop of signal volume and loss of high frequency details of the audio signal coming from the microphone. So, this is impedance mismatch problem.
The solution to impedance mismatch is to use the bootstrap circuit. The bootstrap capacitor C2 feeds back the signal back to the junction of the biasing resistors R1 and R2. The voltage on both sides of the resistor moves together. Since there is almost no voltage difference across the resistor, almost no current flows. The result is the audio source, microphone, sees and input impedance of 1Mohm or more. The result is no or less loss of signal energy and no loss of frequency content of the audio signal and audio without muffled sound; lost treble is produced.
The following video shows via simulation how the bootstrapping circuit works.
In the video, we revisit a classic Class AB audio amplifier design and show how adding a bootstrapped emitter follower dramatically improves sound quality.
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