A Short College Paper About The Workings of The Direct Coupled Amplifier

The direct coupled amplifier is a circuit whose main purpose is to amplify a voltage. Amplification is a process by which a voltage, current or power is increased. Transistors are the components used in the circuit for amplifying the alternating current (AC) signal. Beta gives the transistor its ability to amplify. Beta is the ratio that shows how much larger the collector current (IC) is than the base current (IB). Before amplification can be achieved, the transistor must be properly biased. This means getting the circuit ready to do work by setting the direct current (DC) reference voltages. The collector voltage (VC) is the single DC voltage that best indicates that the circuit is biased properly.

The VC should be approximately one half the DC supply voltage. This value is chosen so that the collector voltage can be made to vary around its resting point to produce an AC output. This circuit is very popular in that it uses feedback to improve temperature stability. A temperature stable circuit is one that is not affected by heat. This characteristic is achieved by designing the circuit to make use of feedback.

The operation of the Direct Coupled Amplifier can be followed step-by-step. First, realize that the circuit is only biased because transistor one (Q1) is connected to a voltage divider formed by resistor one (R1) and resistor two (R2). This voltage divider receives power from the emitter resistor (RE) of the second transistor (Q2).

When the circuit is first powered there is no current flowing through Q2. This means that there is no voltage across RE. And since the voltage drop of RE powers the voltage divider for Q1, Q1 is off. With no current flowing through Q1, there is full potential across VC1. This high voltage on Q1's collector kicks on Q2. This creates A current flow which produces a voltage drop across RE. This voltage powers the divider and turns on Q1. With the increase in current for Q1 there is a high voltage across RC1. The more the current increases the higher the voltage across the collector resistor (VRC) gets. The rising VRC of Q1 lowers the base voltage of Q2. The more Q1 turns on the more Q2 turns off. This happens until the circuit stabilizes. Emitter voltage (VE) is important because it is the value that should be read across RE2 when the circuit is stable, showing that the circuit has been biased properly. Since the emitter voltage of Q2 is always .7 volts less than its base voltage when the circuit is stable, the base of Q1 is held near .7 volts (v).

With the circuit operating properly assume that Q1 starts to heat up. When Q1 heats up the current passing through it increases. This current increase will cause VRC1 to increase. This controls the base voltage of Q2 which decreases. Since the emitter of Q2 is always .7v less than the base, this also causes VRC to decrease. Decreasing the emitter voltage of Q2 will then cause the base of Q1 to decrease. This overall balancing of current results in a smaller heat increase than it would have been if there was no feedback.