Why does Darlington pair have high input impedance?
6 The Darlington Pair. The effect of a high input impedance is to reduce the input current to the amplifier. If the input current for a given input voltage is reduced by whatever method, the effect is to increase the input impedance.
What is input impedance of transistor?
Input impedance is defined as the ratio of imput voltage to input current. It is calculated from the AC equivalent circuit as the equivalent resistance looking into the input with all current cources replaced by an open and all voltage sources replaced by a short.
How to determine the impedance of a Darlington pair?
Determine the input impedance: This is the emitter resistor times the current gain, in parallel with the lower bias chain resistor, in parallel with the upper bias chain resistor. Determine the input capacitor value: The reactance of the input capacitor should be the same as the input impedance at the lowest frequency for a 3 dB roll off.
What are the advantages of using Darlington pair transistors?
Darlington pair has several advantages as compared with standard single transistor. Some of them are It offers a very High input impedance or good impedance transformation that it can alter a high impedance input or source to low impedance load. These can be made by two separate transistors or comes with a single package.
What is the current gain of a Darlington pair?
If the current gain of a transistor is β1 and β2, the overall current gain of Darlington pair is β1β2. The current gain of this transistor is very high compared to the normal transistor.
Why is the leakage current high in the Darlington pair?
The overall leakage current is high because the leakage current of the first transistor is amplified by the next transistor. That’s why the three or more stages of Darlington is impossible. Hence, the Darlington pair is very useful in most of the application because it provides high current gain at low base currents.