transistor uses

[Kevin Weddle]

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.

 

[Kevin Weddle]

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.

 

[Kevin Weddle]

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.