High gain amplifier composed of transistors and field effect tubes

In an ordinary transistor RC coupling amplifier, even if a relatively high collector load resistor Rc is selected,
after connecting to the subsequent stage circuit, due to the low input impedance of the latter stage circuit, high equalization cannot be obtained. Effective AC load resistance
shoes. Therefore, it is impossible to obtain a high voltage gain Kv. If a transistor and a field effect transistor are combined, this
shortcoming can be overcome and a higher voltage gain can be obtained. As shown
    in Figure 8-70, VTi uses a low-noise ultra-Lu transistor , and
its ^FE>IOOO can obtain a lower voltage gain. noise and higher
input impedance. It works in a small current state, and its working c is about
40tcA, so R c1 can take a high resistance value of 300kfl. Since
VT2 is a field effect transistor , its input impedance is very high and will not reduce
the equivalent load resistance of VTi. VT3 is an emitter follower
, used to reduce the output impedance of the amplifier and improve the load
capacity. The two-stage amplifier composed of VTi and VT2
has a high open-loop voltage gain. In the circuit, Rp is a DC
negative feedback adjustment potentiometer, and RF and its 33teF
capacitor in series form an AC negative feedback. The amplifier has stable gain
and DC working status. Its total gain Kv= (RF+ RE) is E. Changing RF can adjust Kvo and Rp so that
the emitter voltage of VT3 is 10V. The amplifier is in the normal working state. The stability of the circuit is quite good, and there is no need to connect a high-frequency
compensation capacitor. Can work in the frequency range of 5 ~ 500kHz. However, its input signal should be small.