Bias Circuit Similarities Vacuum Tube and Transistor Amplifiers

The purpose of any amplifier is to produce a larger output signal from a smaller one. Fig. 1 shows a simple three-electrode vacuum tube amplifier circuit. Both the vacuum tube triode and the soild state semiconductor transistor are three terminal amplifier devices. For proper operation the class-a vacuum tube amplifier circuit requires three different voltage sources marked on the schematic as "VA" filament or heater battery, "VB" plate or anode battery and "VC" as grid or bias battery.

Fig. 1. Basic Triode Amplifier Bias

The "C" battery voltage is selected to provide a constant flow of baseline or quiescent DC plate-current. As shown on Fig. 1, the output AC signal is taken off from the AC_OUT terminal on the right and the input AC voltage is applied via the C_IN coupling capacitor in the left. Of the three electrodes of this triode the cathode terminal is common to both the input and the output circuits and it is connected to ground.

Data sheets provide the required grid voltage value for vacuum tubes. The proper amount of the tube's plate-current is dependent of the bias voltage of the tube used. Vacuum tube bias voltage is usually set to a negative value to stop grid current flow. If a vacuum tube is connected like in Fig. 1 with the grid terminal open circuited, the value of the plate current would reach it's maximum value, limited only by the R_L load resistor.

Fig. 2. Basic Transistor Amplifier Bias

Superimposing the input AC signal on this grid voltage would result in a varying larger anode voltage across the load resistor R_L. If the bias voltage were selected to be -5 volt with a 4 volt AC peak-to-peak input voltage superimposed on it then the grid would swing from -3 to -7 volts. If the tube has amplification rating of twenty than the AC output signal swing would be twenty times higher or 80 volts peak-to-peak.

Because the transistor has no filament and the "VA" battery is not required that result in a cooler operation and a jump of efficiency. The "VB" battery here supplies the collector current and the "VC" supply provides the transistor bias current. The collector voltage is also a lot smaller than in a vacuum tube circuit. If the base terminal is left open and receives no bias voltage in a transistor circuit no current will flow.

For proper circuit operation in Fig. 2, as the circuit uses a NPN transistor, a positive voltage is required on the base and on the collector terminal. By applying a small positive base current to set-up a quiescent current, the superimposed input signal would vary the transistor's input current. If the transistor has a gain of twenty it's output voltage variation would be twenty times larger than that of the input signal. What takes place in both amplifier devices is that their conduction current is modified by the input signal. The main difference between the vacuum tube and the transistor is that the tube is a voltage operated and the transistor is a current operated device. Otherwise they both work quite similarly.