180 Chapter 7
The penalty in this case is that, because such transistor types are not specifi ed for low
noise operation, some preliminary selection of the devices is desirable, although, in
the writer’s experience, the bulk of the devices of the types shown will be found to be
satisfactory in this respect.
In the circuit shown in Figure 7.9 , the input device is used in the common base (cascode)
confi guration so that the input current generated by the PU cartridge is transferred
directly to the higher impedance point at the collector of this transistor so that the stage
gain, prior to the application of negative feedback to the input transistor base, is simply
the impedance transformation due to the input device.
200 μA0.1μF 470 μF47 μF4K71nF330 R330 R2500 μF18 R 18 R18 R^60 30
(Gain)BD435
Input820 R470 μF3K31K5 4K7BC184BC2141.3 mA1.5 mA3VOutput0V
Figure 7.9 : Cascode input moving coil head amplifi er.