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TRANSISTOR ATTENUATOR?


prateeksikka
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Sorry Kevin,
Your circuit is a nightmare. Only a single resistor is a positive supply for 2 transistors. If the input voltage becomes high enough then the PNP transistor's base-emitter junction will become reverse-biased beyond its ratings (only 5V to 7V) and have avalanche breakdown.

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I just wanted to clarify another point. We are so accustomed to using this principle that we forget what the situation is about. Whenver your dealing with a signal and a DC supply voltage, it's easy to say that the change in voltage over the impedance is the change in current. That is with a DC supply.

In the circuit I posted, If you want to know the collector voltage of the PNP it's the change in current times the resistor. If you want to know the base voltage it's the change in current times the parallel combination of the base resistors.

Now the emitter is different. Since there is an input signal and not a DC supply, the change in current becomes the peak to peak voltage across the resistor divided by the resistor, not the change in voltage over the impedance is the change in current as seen from the emitter of the PNP. The signal originates at the collector of the NPN and divides down. A little different but confusing nevertheless. So it's always best to use the peak to peak voltage across the resistor and not just the impedance to get the change in current.

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Kevin,
What are you talking about?
The signal at the collector of the NPN transistor is small and is simply divided by the voltage divider of its collector resistor and the emitter resistor of the PNP transistor.
The collector of the NPN is a high impedance (just the collector resistor) and the emitter of the PNP is a very low impedance.

Why not just make an ordinary transistor amplifier so that you don't have to throw signal away in a voltage divider? ???
Even better, why not just use an opamp? ???

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Hi guys. I just wanted to share the problems I was having with theoretical design. I have something interesting that you might like. Suppose the signal was applied to the base of the PNP. And suppose a much smaller signal were applied to the base of the NPN. What I am getting at is that sometimes the impedance of the collector of a transistor can be lowered. I was thinking that maybe the impedance was the change in voltage over the change in current. But of course it is only that way under certain conditions, signal divided to the DC supply.

I hate to bring about the topic of signal mixing as it really complicates things. But what I want to do is show that you can lower the impedance of the collector.

It's not all that funny that I thought the impedance of a circuit was always the change in voltage over the change in current. And what is really isn't funny is that you can design a circuit based on a false assumption with the right resistors. So basically I overcomplicated this circuit originally, but was able to make it work with the right resistors. Never did I know that book examples could be so misleading until I tried to design a circuit.

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