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The transistor can be used for many things, yet it operates one way. It can be operated as a linear amplifier. It can be operated at cutoff and saturation. It can be used as a resistance, two diodes, or a voltage converter. It may act as a constant current sink or source. Which funtion do you think might work best?

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  • 2 weeks later...

I think it could be used to replace the transistor in some cases. The transistor will not change it's current with a changing collector voltage, but maybe a zener with a diode will. This is certainly a legitmate use that may act similarly to a transistor if the zener and diode were chosen properly. I have no doubt that there exists a circuit which has this combination. The question is, how well will it mimick the transistor. Does anybody follow this assertaion?

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The reason I thought of this is because I wanted to operate a transistor backwords in a circuit. I wanted the collector to act like an emitter. The nice thing about the collector is high impedance. The nice thing about the emitter is the ability to change the current and pass the signal. You will find that the emitter impedance is typically low because the base current. In other words the two resistors at the base are of small value because the base get's it's current through these two resistors.

As far as the characteristics, I would guess the zener would have to conduct quite a bit when the voltage at the base changes. The zener, under normal conditions, conducts more when the voltage across it exceeds the rated voltage.

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I don' know. Do you have the idea that this will work to a certain extent. It becomes a little tricky when you get away from the transistor model, add a 5 layer device it's even more complicated. Maybe as far as the hfe goes, it would depend on the zener conducting heavily with a small change in base voltage. The diode would have to conduct heavily too or else the base current would be too high. You need current to flow from collector to emitter.

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The high impedance, but able to pass the signal. You will find that the two base resistors have to be low value in order to get the base current. This makes the emitter impedance low. I needed a higher impedance for the purpose of gain.


In that case, you might want to look into using a FET.
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I underatand what you are saying, but a FET gives you the same result. A change in voltage divided by the change in current gives you the impedance. The question is , how much change in voltage will give you the change in current. With a FET, it's dependent on the gm. Maybe the gm could be any value, but I bet the gm is always high because of the biasing. This means a small change in voltage will give you a large change in current. A small gm would be ideal. Just like a reduced beta would be ideal. But a reduced beta is found with high current transistors which makes the two base resistors even lower.

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  • 1 month later...

Sec has the exact quote that I intended to be illustrated. The emitter impedance has to be low because of the the two base resistors. You can't just choose high value resistors and expect to get the high impedance. The transistor dictates the value of these two resistors. This is why a need a special type of transistor. I can in effect make the change in voltage over the change in current independent of the base resistors as far as the impedance.

Let's say the change in voltage over the change in current is 1kohm. The impedance is then 1kohm. But the change in voltage causes a greater change in current at the other end. This way, I have a 1kohm impedance with a change in voltage that causes a greater change in current at the other end of the device.

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Does anybody follow the assertion that you can have a change in voltage divided by the change in current produce a change in voltage divided by a greater change in current. This is what a tranistor emitter follower does. But I'm looking for a three terminal device that will use use a common current branch. The change in voltage produces a change in current at one end while I get a greater change in current at the other end. This is for increasing the impedance. This would be a common base type layout with the output being a low impedance. Something like this:


100Vpp 100Vpp
-------10kohm--------device---------1kohm
change change
of of
10mA 100mA

The question is, where is the increased current going to come from? I would have to add a supply voltage on the output. This is just an emitter follower, except the change in voltage is too high. If I were to use the common base configuration, I would have an increase in voltage causing an increase in voltage and the VCE would stay constant. This is the advantage of using a large signal with a common base configuration. The emitter follower will produce an increase in voltage with a decrease in voltage thus changing the VCE too much.

Why can't I use a common emitter setup? Because I want the signal to be able to pass from both directions. You can apply the signal at the base or the emitter of a transistor, but not the collector.

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