This is a circuit that is design for feedback the amplifier. The circuit is used to achieve a specified amplifier gain we usually use op amp amplifier design to ease the setting of the gain. This is the figure of the circuit.
When the gain accuracy is not critical, we can use transistor feedback amplifier as the base of our amplifier design. This circuit is built by transistor. Without TR2, the circuit is taking the emitter of TR1 as the output then we can see an emitter follower circuit, the voltage variation if TR1 base cause the emitter voltage to follow the base voltage. The mechanism of the emitter follower is based on the fact that the small base-emitter current will cause a large collector- emitter current, but this current cause the emitter voltage (the voltage across the emitter resistor) to rise, and the this reaction will make the base-emitter current to decrease since the current is the result of base-emitter voltage difference.
This action and reaction is the basic of the feedback mechanism in the circuit. Now look at the TR2, this transistor amplify the collector current of the TR1, and now the feedback source is supplied from the TR2 collector through R4. TR2 make the collector current of TR1 very small, since the feedback is now handled by TR2. The current drawn by the base of TR1 becomes smaller, providing batter amplifier impedance at the input.
When the gain accuracy is not critical, we can use transistor feedback amplifier as the base of our amplifier design. This circuit is built by transistor. Without TR2, the circuit is taking the emitter of TR1 as the output then we can see an emitter follower circuit, the voltage variation if TR1 base cause the emitter voltage to follow the base voltage. The mechanism of the emitter follower is based on the fact that the small base-emitter current will cause a large collector- emitter current, but this current cause the emitter voltage (the voltage across the emitter resistor) to rise, and the this reaction will make the base-emitter current to decrease since the current is the result of base-emitter voltage difference.
This action and reaction is the basic of the feedback mechanism in the circuit. Now look at the TR2, this transistor amplify the collector current of the TR1, and now the feedback source is supplied from the TR2 collector through R4. TR2 make the collector current of TR1 very small, since the feedback is now handled by TR2. The current drawn by the base of TR1 becomes smaller, providing batter amplifier impedance at the input.
