Audio Amplifiers

Volume Controls

The current, voltage, and power gains of transistor stages are usually varied by using variable resistors in the coupling networks. The variable resistor is usually referred to as a gain control. At audio frequencies, the gain control is referred to as a volume control.

The volume control and its associated circuit should permit the variation of gain from zero to maximum. Moreover, the volume control should be so arranged in a given circuit that all frequencies are attenuated equally for all positions of the variable arm of the control.

A large number of satisfactory volume control circuits can be designed. A typical volume control circuit is discussed below.

The two-stage amplifier (see the figure below) uses variable resistor R₂ as a volume control. Collector load resistor R₁ develops transistor Q₁ output signal. DC blocking capacitor C₁ couples the signal to variable resistor R₂ and the transistor Q₂ input circuit. Resistors R₄ and R₃ form a voltage divider to establish the base bias voltage. Resistor R₅ is the emitter swamping resistor and is AC bypassed by capacitor C₂. Collector load resistor R₆ develops the output signal. Placement of variable resistor R₂ in the circuit as shown is unsatisfactory for the reasons stated below.

(1) The base bias voltage is that voltage developed across resistor R₃. This voltage will vary from a maximum to zero depending on the position of the variable arm on variable resistor R₂. The voltage developed across resistor R₃ must cancel the opposing voltage developed across swamping resistor R₅ and leave a net forward base-emitter bias. This condition will not be met with variable resistor R₂ shunting resistor R₃.

(2) The DC current flowing through variable resistor R₂ will cause excessive noise.

(3) The gain of the low frequencies will vary according to the position of the variable arm on variable resistor R₂. This condition can be understood by considering the base-emitter junction resistance of transistor Q₂ as negligibly small. With this assumption, variable resistor R₂ can be considered as shunting emitter swamping resistor R₅. With all of the resistance of variable resistor R₂ in the circuit, capacitor C₂ effectively bypasses the low frequencies around the parallel combination of resistor R₅ and variable resistor R₂ to minimize degeneration of the low frequencies. As the variable arm moves to reduce the portion of variable resistor R₂ in the circuit, capacitor C₂ becomes less effective as a bypass capacitor for the low frequencies, causing greater degeneration of the low frequencies compared to the high frequencies.

The circuit shown in the figure below avoids the unsatisfactory conditions found in the circuit shown in the figure above. The functions of the parts of the two circuits bearing the same reference symbols are the same. DC blocking capacitor C₃ has been added to the circuit of the figure below.

Capacitor C₃ avoids the variation in base bias voltage by DC isolating resistor R₂ from resistor R₃. The noise factor of the amplifier is also improved by preventing DC current flow through the variable resistor.

By connecting the lower end of variable resistor R₂ to the upper end of resistor R₅ and capacitor C₂, variable resistor R₂ does not AC shunt resistor R₅ and capacitor C₂. This arrangement avoids the deterioration of the low-frequency response of the amplifier as the resistance of variable resistor R₂ is reduced.