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# Input and output resistances for an amplifier

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#### Gupsh

##### Junior Member level 2
how input and output resistances characterise the amplification quality of an amplifier? It may sound a little bit novice to ask but I am really truing to know all about amplifier so please explain.

#### chuckey

The input and output resistance of an amplifier, should be a good match for the signal source and the signal destination. e.g. if you are using a crystal microphone into an amplifier that is driving a loudspeaker. the crystal microphone can deliver a good voltage (.2V) but only at a very low current (.2 micro amps), if your amplifier has an input impedance of IK ohms, then it will get the .2 micro amps, which will correspond to an input voltage of . 2 X 10^-6 X 1 X 10^3 = .2mV. if the amplifier had an input impedance of 10 M ohms then the input signal would be (10/10+1) X .2V ~ .18V. So having an amplifier with a high impedance input is of great value if the signal source is high impedance.
Frank

#### Ow@i$##### Advanced Member level 1 your output and input resistance should match the output and input source/load... that's the key to achieve max power transfer and efficiency #### LvW ##### Advanced Member level 5 your output and input resistance should match the output and input source/load... that's the key to achieve max power transfer and efficiency Yes, agreed - however, only in case your goal is max power transfer. However, my impression is that the questioner needs a voltage amplifier!. #### Ow@i$

Yes, agreed - however, only in case your goal is max power transfer. However, my impression is that the questioner needs a voltage amplifier!.

correct me if i am wrong but there is a misconception that power amplifier amplifies power in fact it either amplifies current or voltage whose product at output and input determines Power in and Power out!!!

matched load therefore ensures no reflection of either current or voltage back... !!!
correct me if i am wrong

#### chuckey

"matched load therefore ensures no reflection of either current or voltage back... !!!
correct me if i am wrong"
All HI-Fi amplifiers have an output impedance of about .1 -> .02 Ohms, All Hi-Fi loudspeakers have an impedance of 3 -> 15 Ohms. I think you stand corrected!
Frank

#### godfreyl

I think smoothcriminal's talking about impedance matching as used for RF and digital equipment, in which case what he says makes sense.

E.g. a signal is sent from a device with an output impedance of 50 ohms through a cable with a characteristic impedance of 50 ohms to a device with an input impedance of 50 ohms.

It's also important for long distance telephone lines to prevent echos (as sometimes heard on international calls).

With hifi equipment it's not important as the cable length is much shorter than the wavelengths of the signal.

#### Ow@i$##### Advanced Member level 1 yes for sure i was talking about RF equipment....!!! however i still wanna know that power amplifiers are behind the curtain doing either voltage or current amplification .. and not exactly Power amplification!!! am i right !! godfreyl & chuckey ??? #### sky_123 ##### Advanced Member level 4 By definition an amplifier must increase power (which is IV) - a transformer can be used to raise voltages to high levels but a transformer is not an amplifier.. #### Ow@i$

@sky_123
so you mean to say in power amplifier there is a transistor configuration that is amplifying both current and voltage?? :-? and what exactly this configuration is ??? :-?

#### LvW

here is an excerpt from

https://www.electronics-tutorials.ws/amplifier/amp_1.html

As their name suggests, the main job of a "Power Amplifier" (also known as a large signal amplifier), is to deliver power to the load, and as we know from above, is the product of the voltage and current applied to the load with the output signal power being greater than the input signal power. In other words, a power amplifier amplifies the power of the input signal which is why these types of amplifier circuits are used in audio amplifier output stages to drive loudspeakers.

The power amplifier works on the basic principle of converting the DC power drawn from the power supply into an AC voltage signal delivered to the load. Although the amplification is high the efficiency of the conversion from the DC power supply input to the AC voltage signal output is usually poor. The perfect or ideal amplifier would give us an efficiency rating of 100% or at least the power "IN" would be equal to the power "OUT". However, in reality this can never happen as some of the power is lost in the form of heat and also, the amplifier itself consumes power during the amplification process. Then the efficiency of an amplifier is given as Vout/Vin.