Is there a logical understanding of I/P and O/P resistances of various amplifiers?

Note: the ideal values of resistances would be (infinite for high and zero for low)

the voltage amplifiers gain is = V out / V in

the current amplifiers gain is = I out / I in

the trans resistances amplifiers gain is = V out/ I in

the trans conductance amplifiers gain is = I out / V in

Observations made but don't know the reason why

1)When voltage is present at the input, the input resistance would be high and when voltage is taken across the output, the output resistance would be low (for voltage amplifiers)

2)similarly, when current is at the input the input resistance would be low and when voltage is taken across the output, the output resistance would be high(for current amplifiers)

combinations of these 2 points have made up the table for the other 2 cases...(resistance and impedance amplifiers)

If anybody could provide insight or clarify why these above 2 points are what they are... that would be so helpful!!

Thanks & Regards
Sunil

Hi,

I assume it´s more obvious if you start with the problem than with the solution.
Then decide the requirements and find out that your table makes sense.

Although you could build a voltage amplifier with low input impedance, too - it doesn´t necessarily need high imedance.

****
An example:
microphone --> audio amplifier --> speaker.

* A speaker usually is designed to operate "voltage driven". Usual speaker impedance is in the range of 8 Ohms.
* Microphone: There are different ones. Let´s assume to use a dynamic microphone. It converts the audible sound into an electrical signal. The acoustic power that hits the microphone is limited. The generated electrical power must be less than the acoustic power, because efficiency is always below 1. It usually is in the range of microwatts, the source impedance maybe in the range of 1k Ohms. Thus the voltage is also limited to some 100mV.

For the amplifier input this means: if the amplifier input impedance is too low (<<1k Ohm) then the voltage drops and you need a higher gain amplifier. The higher the gain the more problematic the the signal bandwidth. Thus one wants relatively high amplifier input impedace. Usually >> 1k Ohms.
(There may be benefits regarding noise, frequency response, resonance when the amplifier input impedance is close to the microphone source impedance)

For the amplifier output this means:
For a speaker to be "voltage driven" the amplifeir output impedance must be less than the speaker impedance. The less the better.

--> Thus for an audio amplifier the requirements are:
* A voltage amplifier
* input impedance >> 1k
* output impedance << 8 Ohms

Other applications will have other requirements, resulting in other amplifier types. Like your table shows.

Klaus

It all boils down to understanding how a voltage source or current source works and the effects of loading
Recall that a practical voltage source has low source resistance and conversely a current source is characterized by high impedance.
Also recall that in measuring a voltage with a tester you place the probes at each end (in parallel) while for a current you place the probes in series.
For an accurate measurement for a voltage, the internal resistance (in parallel) of the tester must be as high as possible while for accurate measurement for a current, the internal resistance (in series) must be as low as possible.

This principle also applies for amplifiers.