Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Basic Electronics
The Two-Transistor Analogy
In this analogy, the collector region of the n-p-n is in common with the base of the p-n-p, and the base of the n-p-n serves as the collector region of the p-n-p. The center junction j2 serves as the collector junction for both transistors.
The Two-Transistor Analogy:
- This two-transistor analogy is illustrated in Fig.
- The collector current ic1 of the p-n-p transistor drives the base of the n-p-n, and the base current iB1 of the p-n-p is dictated by the collector current iC2 of the n-p-n.
- If we associate an emitter-to-collector current transfer ratio a with each transistor, we can use the analysis to solve for the current i.
with α1 = αN for the p-n-p, α2 = αN for the n-p-n, and with ICo1 and IC02 for the respective collector saturation currents, we have
But the sum of iC1 and iC2 is the total current through the device:
Taking this sum in the above Eq. we have
As this equ indicates, the current i through the devices is small (approximately the combined collector saturation currents of the two equivalent transistors) as long as the sum α1 α2 is small compared with unity.