impedance of amplifier

[Bhuvanesh123]

i am studying bjt amplifiers,i am often getting through impedance of input and outputs . Its messing up with me
example:common collector configuration have high input impedance and low output impedance
does that input impedance is impedance between the terminal( base and collector)?
it is also said as impedance matching circuit.why common base and common emitter are not being said impedance matching circuits.Thank you in advance

 

[BradtheRad]

common collector configuration have high input impedance and low output impedance
does that input impedance is impedance between the terminal( base and collector)?

This mode has the emitter resistor. Output is taken at the transistor's emitter terminal.

Any C-E current raises the voltage on the emitter resistor. This creates the need for greater base voltage.

Input current (B-E) must travel through the emitter resistor. This has the effect of raising input impedance.

B-E is just one portion of input impedance.

The comparison tables state input impedance as higher than common-base or common-emitter configurations. I think this means it can be high for normal operation, but it does not necessarily need to be high.

 

[D.A.(Tony)Stewart]

Current gain transforms to impedance between base and emitter by the current gain in each direction.

Thus CE has input impedance of β*Ze where Ze is the impedance on the emitter ( RLC)
and the output impedance looking at emitter is β*Zb , where Zb is the equivalent impedance applied to bias or filter into the base.

But since Ie≈Ic the impedance , there is no current gain but voltage gain is the impedance ratio of Rc/Re where Re is the source driving the emitter.

If the base impedance, bypass cap, β (or implied transimpedance, gm )and Ie all affect the low emitter input impedance so it usually sourced from a low impedance source like 50 Ohms.

A differential Amp has an equivalent circuit of a CE driving a CB so the output to increase Vgain, Add becomes -Rc/Re but has high input impedance.

In all cases the internal base-emitter resistance is assumed to be lower than re but if not <<Re then it is included, which is normally in the < 10 Ohm range depending on bias current.

The only configuration that inverts voltage with respect to ground is Base to collector or CE .

 

[vGoodtimes]

The common collector has current gain, but no voltage gain. It provides a high input impedance to the input, and a low output impedance to the output.

Most circuits have specific voltage input/output requirements. The current aspects to these designs is then determined from these original voltage-based requirements.

Common base is really the "dual" of common collector in that it provides a low input impedance and a high output impedance. It has no current gain, but does have voltage gain.

For the same reason you could argue common base is also impedance matching for current input/output designs. However, this would most likely hinder communications with other engineers who view "impedance matching" as being a more specific concept.

Common emitter is really what I think of when I think of "impedance matching" as you can get specific input and output impedances (vs just high/low). This would be closer to the RF designs where source/line/load are designed to have the same impedance.

I don't use the term "impedance matching" outside of the RF world, as the term is not clear.