Dynamically biased high-fidelity power amplifier circuit (b)

There are many methods of dynamic biasing. The method used here is shown in Figure 2-123(b). When static, the output stage bias
voltage is provided by VTi and VTz. VT3 passes through VI island and R3. VT4 passes through VI island and R It is in a micro-conduction state and
has a great impact on VTi VT2. When a signal arrives, in the positive half cycle of the signal, because the emitter-to-ground potential of VT3
increases as the signal increases, VT3 will turn from the micro-conduction state to the full conduction state, causing the bias voltage of VTi to become smaller.
The voltage cel between the collector and the emitter increases. Set the parameters of VTi appropriately so that 'cel is greater than the signal voltage. At this time, VT2 is still
in a static bias state, and the potential of point B is not higher than the static potential. In the same way, in the negative half cycle of the signal, Uβ is not less than the signal
voltage, and the potential of point A is not lower than the static potential.
    From the above analysis, it can be seen that whether it is the positive half cycle or the negative half cycle of the signal, VAB is always greater than the signal voltage, so that
the two arms of the output stage will never work in the cut-off state. In order to further reduce the crossover distortion, the output can be adjusted by adjusting RPi The high
quiescent current of 200mA is beneficial to the amplification of small signals
, reduces the action of dynamic bias, and can reduce the impact of dynamic bias lag. This is also a feature of this dynamic bias circuit.
    For dynamic bias For the selection of bias circuit components, VTt~VT4 should use high-frequency, low-noise, low-power tubes with a withstand voltage greater than
60V, and RPi should use double-connected potentiometers to ensure synchronization of adjustments.