Negative feedback is supplied by R5 and R2, which guarantee stability by minimizing the gain marginally. Smaller value for C2 capacitor can cause a weak reduced frequency response. The signals are mixed at the the common emitter joint of Q2 and Q3, and transferred to the loudspeaker by means of the big electrolytic capacitor C2. If the amplified input signal is no more than 4V5, Q2 is switched off (since the base will likely be at the reduced voltage compared to its emitter), nevertheless Q3 may continue to pass the signal.Īs soon as Q1 amplifies the signal over 4V5 the situation gets reversed, Q2 switches ON and Q3 is switched off. Within the intersection of the Q2, Q3 emitters the voltage may also be almost 4V5, R3 and R4 and extremely small value resistors to control the current passing across Q2 and Q3. The Q2 and Q3 bases are likewise with the identical voltage (pretty much) as the Q1 collector due to the fact the R1 value is very small (68R). The collector voltage of Q1 is around 50 % of the supply voltage, i.e. The collector load for Q1 consists of R1, R5 along with the loudspeaker. The input signal is transferred via the volume control RV1 and next through C1 to the Q1 base. Power output is around 1W, with fairly minimal distortion. The first mini audio amplifier circuit works with a 'complementary' output stage, having a single NPN and a single PNP power transistor, that gets rid of an output transformer commonly observed in older amplifier models.