Input and Output impedances of Current and Voltage Amplifiers

Question

I read that Voltage Amplifier have high input impedance and low output impedance, and Current Amplifier have low input impedance and high output impedance. If this is true what is reason for this as I could not find one. FETs have high input impedance and can act as voltage amplifiers, I think this confirms that voltage amplifiers have high input impedance.

On the other [hand] when we come to Common Collector and Common Base Amplifiers, the former one high input impedance but have voltage gain of less than 1. Is that not violate the basic property voltage amplifiers?

Since it have high input impedance, should it not have high voltage gain?

Similarly the later one have low input impedance but have significant voltage gain and no current gain, again opposite to the property of current amplifier?

Answer 1

I read that Voltage Amplifier have high input impedance and low output impedance, and Current Amplifier have low iremainnput impedance and high output impedance.

These are definitions that we impose on the circuits to make them easier to think about, so it's dangerous to try to make sweeping generalizations from real circuits. An amplifier is a voltage amplifier if the designer intended that it respond to voltages with voltages, etc. But one could conceivably have any topology be a "voltage" or a "current" (or transresistance or transimpedance) amplifier.

Answer 2

On the other [hand] when we come to Common Collector and Common Base Amplifiers, the former one high input impedance but have voltage gain of less than 1. Is that not violate the basic property voltage amplifiers?

No, voltage amplifiers can have gains that attenuate. This can be done with most op amps. Typically it doesn't make sense to use op amps as attenuators because it can also be done with only resistors. If you need impedance matching then use a voltage follower.

Since it have high input impedance, should it not have high voltage gain?

Input impedance doesn't have anything to do with the gain of the amplifier, a voltage amplifer can have a gain of 1 and have very high input impedance. It's the feedback network and open loop gain that determine the gain of an amplfier. However if you want high gains then it becomes necessary to have high input impedance when amplifying low level signals (like nV's) .

Answer 3

When you consider a single transistor is a custom amplifier that is usually just one of many or in an IC with many transistors.

There are many "standard" configurations that are tradeoffs:

1. Voltage gain : Collector output
2. Current gain ; Collector or Emitter Follower (voltage gain =1 )
3. Conductance gain = impedance reduction
4. Power gain (any combination of the above )
5. Linearity improvements : using negative feedback, which lowers input AND output impedance but reduces voltage gain by using R ratios
6. Bandwidth (fixed GBW)

7. Signal inversion (collector) or not (emitter)

   • If you want to maximize any of the above (1~4) then there must be compromises in some others.

   • To get more voltage gain, the input impedance may be lowered or output impedance raised to achieve this.

The current gain can vary widely and also depends on collector current. By using current gain with resistor or "impedance" ratios, we get voltage gain.

Bypass capacitors make use of low DC gain circuits but high AC gain.

There are constant voltage circuits and also constant current circuits with no gain but just used as regulators and there may be many of these current regulators inside linear ICs' to raise impedance and keep current constant independent of hFE.

There are also other amplifiers such as Gyrators which use a negative impedance characteristic and others simply with negative feedback e.g. convert a capacitor into inductor behaviour.

Answer 4

This is a fine voltage amplifier, yet Rin is 2.6 ohms (0.026v/0.01amp).

^{simulate this circuit – Schematic created using CircuitLab}