Most Hi Fi amplifiers are similar to this design. The circuit uses a complementary output stage with one NPN and one PNP power tran­sistor; this eliminates the need for an output transformer. Output power is close to 1W with reasonably low distortion. The input signal is passed through the volume control RV1 and then via C1 to the base of Q1. The collector load for Q1 is com­posed of R1, R5 and the loud­speaker and the voltage at the col­lector will be about half the supply voltage, i.e. 4V5.

Circuit in lines (Click to enlarge)

The bases of Q2 and Q3 are also at the same voltage (very nearly) as the col­lector of Q1 because the value of R1 is so low. At the junction of the emitters of Q2 and Q3 the voltage will also be very nearly 4V5. R3 and R4 limit the current through Q2 and Q3. When the amplified input signal is less than 4V5, Q2 is turned off but Q3 will conduct. When Q1 ampli­fies the signal to above 4V5 the reverse happens; 02 conducts and Q3 is turned off.

The signals are combined at the common emitter junction of Q2 and Q3 and passed to the loud­speaker through the large electro­lytic capacitor C2. Small values of C2 result in a poor low frequency response. Negative feedback is provided by R5 and R2; these en­sure stability by reducing the gain slightly.

R1 is included to provide a small amount of base bias for G2 and Q3; more sophisticated de­signs use thermistors or diodes to prevent thermal runaway. A disadvantage is the DC coupl­ing of the transistors; the output pair should be ‘matched pair’, other types can be tried as long as they are also ‘matched pairs’.

Regards

Team Coreelectronics