0

I was reading about hybrid parameters from the text book 'Engineering circuit analysis by Hayt-8th edition'. I read a paragraph saying :-

" The difficulty in measuring quantities such as the open-circuit impedance parameters arises when a parameter such as z21 must be measured. A known sinusoidal current is easily supplied at the input terminals, but because of the exceedingly high output impedance of the transistor circuit, it is difficult to open-circuit the output terminals and yet supply the necessary dc biasing voltages and measure the sinusoidal output voltage. A short-circuit current measurement at the output terminals is much simpler to implement. "

Here, I don't understand the statement " because of the exceedingly high output impedance of the transistor circuit, it is difficult to open-circuit the output terminals and yet supply the necessary dc biasing voltages and measure the sinusoidal output voltage." Can anybody clarify what they are implying?

Screenshot

Lelouch Yagami
  • 397
  • 3
  • 8
  • 1
    It's not saying it's hard to open-circuit the output terminal of the transistor. But if you do, it's not easy to keep the transistor biased in active mode. – The Photon Mar 01 '23 at 06:22
  • What about this part -> " because of the exceedingly high output impedance of the transistor circuit " – Lelouch Yagami Mar 01 '23 at 06:30
  • 1
    Consider the structure of a bias tee. What about it would complicate the measurement of a high impedance? – Tim Williams Mar 01 '23 at 12:00
  • 1
    @LelouchYagami. In order for your connected "open circuit" to be an effective open circuit it must have much higher impedance than the device you're trying to measure. The high impedance of the transistor circuit's output makes it harder to achieve that. – The Photon Mar 02 '23 at 02:47
  • Ok. Got it. Thanks. – Lelouch Yagami Mar 02 '23 at 04:11

1 Answers1

2

Too bias the transistor, a particular dc collector-emitter voltage must be applied. ( for example, 10V). However, for ac signals, the collector and emitter terminals should be open-circuited. So, at dc, a low impedance constant voltage is needed, while for ac signals, a high impedance must be presented to the terminals.

One solution to this would be to connect the collector and emitter to a very large inductor, and then the inductor to a dc power supply. However, it is extremely difficult to make ideal inductors.

Other circuit techniques could be used, but it is difficult to make a high impedance for ac signals with a constant dc bias.

It is easy to make a low impedance at ac with a dc bias, a good power supply does this. Thus, for transistors, it is easier to measure y parameters or some hybrid parameters than z parameters.

user69795
  • 1,517
  • 7
  • 8