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Three Stage Transistor Amplifier

Amplifier circuits can input a small signal, and then multiply its voltage and current output.  The output of the amplifier can be millions of times greater than the input.  This large increase in signal power is derived from the DC power source (Vcc).  A a car audio amplifier outputs only a fraction of the DC power that it draws from the battery as sound.



The first stage of the above amplifier provides high gain.  It converts a very small signal (1mV) to a small signal ( >200 mV).  Since the output signal of the first stage is small, the operating point can be far from the Q point of the circuit.  We can see from the second frame of the animation that the operating point is low ( TP3 = 2.43 volts). This circuit operating point and gain is dependent on the Beta of Q1. This could make it thermally unstable or even require Q1 transistor selection during circuit manufacture. The thermal instability could require tight cooling specification for the circuit.  

The low frequency gain of the Q2 stage is fixed by the ratio of collector resistor to the emitter circuit resistor. An input of 89 mille-volts at 100 Hz is amplified to 481 milli-volts.  At lower frequency the Q1 and Q2 bypass capacitors attenuate the signal to the Q1-Base and Q2- Base. At 1000 Hz and above the base bypass capacitors impedance becomes negligible. Also at higher frequencies, the signal at Q1 and Q2 emitter drops significantly due to the emitter bypass pass capacitors drawing more current.  Above 1000 Hz the bypass capacitor impedance is further reduced, which further improves overall circuit gain.  Above 100000 Hz transistor gain starts to drop significantly due to internal transistor capacitance.  Above 1Mhz transistor and overall circuit gain are dropping off rapidly with increased frequency.  


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