Low noise preamplifier (b)

It is a preamplifier circuit composed of domestic low-noise 2G213 and 2G212. Among them, the Ic of VTi
is 50UA, which is suitable for occasions where the internal resistance of the signal source is lOkfl, such as being used as a playback preamplifier for a recorder. Use
switch SA to change the AC feedback network to obtain the frequency equalization characteristics required for records (RIAA) or tape recording (NAB)
. In the figure, RE affects the equivalent input noise. When RE increases, the noise also increases. If the differential input form is used,
the impact of RE on the noise characteristics can be avoided.
    Figure 8-19(b) is a dynamic microphone preamplifier composed of a common-emitter and common-base circuit . In the figure, VTi and VT2 form a common
-emitter common-base amplifier. The base voltage of oVT2 is biased by the forward voltage of diode VI)1 and vrh (about 1V). Therefore
, the collector voltage of VTi is only about 0.5v, and the collector currents of rl and VTz are only tens of microamps,
so the noise of the amplifier is very low. The collector of VTz uses the input resistor of VT3 as a dynamic load. Resistor, with high
voltage gain (Kv=500). VT3 is an emitter follower. The emitter circuit of VT3 also applies
AC and DC negative feedback to the base of VTi. The input end of VTi communicates with the moving coil through the transformer T. type microphone connection to accommodate
microphones of different impedances. The circuit has very low noise, wide dynamic range and high stability. When a 60plV signal is added to the 50fl input of the input transformer T
, the output voltage can reach 30mVo. In the figure, VTi should use a low-noise transistor, and the ^FE of VT3 is
about 200.