Class A Simple - Audio Amplifer

<div>This circuit is a scaled-up version of the earlier 9 V Class A common-emitter amplifier. The goal was to preserve the same basic amplifier behavior while increasing the supply voltage to 60 V and deliberately biasing the collector near the midpoint of the available voltage range. ### Circuit values #Vs = 60 V #Rc = 45 kΩ #Rb = 9.5 MΩ #Cb = 1 µF #ACs = 1 kHz input signal #Qa = NPN transistor #Vce monitors the collector-emitter voltage. #Vbe monitors the base-emitter voltage of #Qa. Since the emitter is grounded, #Vbe is effectively the same as the base voltage relative to ground and represents the actual control voltage that sets the transistor’s operating point. ### How the values were chosen The original 9 V circuit used a 9 kΩ collector resistor and operated at approximately 0.67 mA quiescent collector current. For the 60 V version, the desired collector quiescent voltage was approximately half the supply: $$V_{CQ}\approx\frac{V_{CC}}{2}$$ Therefore: $$V_{CQ}\approx\frac{60}{2}=30V$$ Keeping approximately the same quiescent current as the 9 V version: $$I_{CQ}\approx0.67mA$$ The required collector resistor is approximately: $$R_C=\frac{V_{CC}-V_{CQ}}{I_{CQ}}$$ $$R_C=\frac{60-30}{0.00067}\approx44.8k\Omega$$ A practical value of approximately: $$R_C=45k\Omega$$ was used. ### Base-bias calculation The previous 9 V version used approximately 1.33 MΩ for the base-bias resistor. The corresponding base current was approximately: $$I_B=\frac{9-0.7}{1.33M\Omega}\approx6.24\mu A$$ To maintain approximately the same base current from a 60 V supply: $$R_B=\frac{60-0.7}{6.24\mu A}$$ $$R_B\approx9.5M\Omega$$ Therefore #Rb was set to approximately 9.5 MΩ. ### Measured result With the circuit running, #Vce showed approximately: Minimum collector voltage: $$V_{C(min)}\approx19.9V$$ Maximum collector voltage: $$V_{C(max)}\approx41.4V$$ Peak-to-peak output swing: $$V_{pp}\approx21.5V$$ The approximate midpoint is: $$V_{CQ}\approx\frac{41.4+19.9}{2}\approx30.6V$$ This is very close to the intended 30 V quiescent collector voltage. ### What is happening The collector voltage follows: $$V_C=V_{CC}-I_CR_C$$ When the input signal causes more base current: collector current increases → voltage drop across #Rc increases → collector voltage falls. When base current decreases: collector current decreases → voltage drop across #Rc decreases → collector voltage rises. This produces the normal 180° phase inversion of a common-emitter amplifier. The input signal is applied through #Cb and appears at the base-emitter junction as #Vbe. Small variations in #Vbe around its DC operating point control the collector current. Even small changes in #Vbe produce relatively large changes in collector current, which is what enables voltage amplification at the collector. ### Result The 60 V version is now biased near the middle of its available collector-voltage range and produces a clean amplified sine wave without obvious clipping. This remains a small-signal Class A voltage amplifier. The next step is to redesign the output stage for much higher current so it can drive a low-impedance load such as an 8 Ω resistor used to approximate a loudspeaker. <br></div>

published 11 hours ago