This amplifier circuit is a bit simplified from what you will normally encounter in practical multi-stage circuits. It is this same principle that is used to make operational amplifier circuits behave so predictably. Thus, it is easier to design an amplifier with predictable gain by building a staged network of transistors with an arbitrarily high voltage gain, then mitigate that gain precisely through negative feedback. This is good, because it is far easier to manufacture consistent resistors than consistent transistors. What effect do you think a decrease in feedback resistance have on voltage gain? What about an increase in feedback resistance? Try it and find out!Īn advantage of using negative feedback to "tame" a high-gain amplifier circuit is that the resulting voltage gain becomes more dependent upon the resistor values and less dependent upon the characteristics of the constituent transistors. Simply put, the amplifier isn't nearly as "touchy" as it was without the feedback resistor in place.Īs with the simple common-emitter amplifier discussed in an earlier experiment, it is a good idea here to make a table of input versus output voltage figures with which you may calculate voltage gain.Įxperiment with different values of feedback resistance. You should notice the reduction in gain immediately by the decreased sensitivity of the output signal on input signal changes (changes in potentiometer position). As such, it will act to stabilize the amplifier's response and minimize the voltage gain. Since the overall response of this three-stage amplifier is inverting, the feedback signal provided through the 1 MΩ resistor from the output of the last transistor to the input of the first should be negative in nature. You may experience these relationships by measuring the collector-to-ground voltage at each transistor while adjusting the input voltage potentiometer, noting whether or not the output voltage increases or decreases with an increase in input voltage.Ĭonnect the 1 MΩ feedback resistor into the circuit, coupling the collector of the last transistor to the base of the first. Since any one of the common-emitter "stages" is inverting in itself, an even number of staged common-emitter amplifiers gives noninverting response, while an odd number of stages gives inverting.
3 RESISTOR NPN TRANSISTOR AMPLIFIER FULL
You may find it impossible to adjust the potentiometer for a stable output voltage (that isn't saturated at full supply voltage or zero), the gain being so high.Įven if you can't adjust the input voltage fine enough to stabilize the output voltage in the active range of the last transistor, you should be able to tell that the output-to-input relationship is inverting that is, the output tends to drive to a high voltage when the input goes low, and vice versa. I recommend building this circuit without the 1 MΩ feedback resistor to begin with, to see for yourself just how high the unrestricted voltage gain is. By connecting three common-emitter amplifier circuit together - the collector terminal of the previous transistor to the base (resistor) of the next transistor - the voltage gains of each stage compound to give a very high overall voltage gain.
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