Amplifiers and their impedance(s)

Thread Starter

Spacerat

Joined Aug 3, 2015
19
Hello Forum,

There are independent and dependent sources (VCVS, CCVS, VCCS, CCCS). A dependent source consists of two parts: controlling part and controlled part. I read that the controlling part is either an open circuit or a short circuit. The controlled part is either a voltage source or a current source.

There are different types of amplifiers:
  • voltage amplifiers: the output voltage is larger than the input voltage
  • current amplifiers
  • power amplifiers
Why are voltage amplifiers supposed to have high input impedance and low output impedance (vice versa for current amplifiers)? What is the rational? A device need to be connected to the input side of the amplifier and another device to the output side of the amplifier. What happens to the current when when the voltage is amplified? Does the current decrease to keep the output power equal to the input power?

In general, a device has just one single impedance and impedance matching is often required to avoid energy reflection at the connection interface between different elements....

I read an article about audio amplifiers and speakers (http://www.the-home-cinema-guide.com/speaker-impedance-matching.html#axzz3mDqxGy7z) where it is said that "...As stated earlier, the amplifier doesn't have an output impedance....It is the speaker that has the impedance....Therefore, the impedance you may see listed for an amplifier refers to the optimum speaker impedance that it is designed to drive....You can connect speakers of any impedance to an amplifier and they will work. However, if the amplifier isn't designed to drive speakers with a lower impedance (4 ohms for example), then the amplifier may overheat if you turn the volume up very loud - because it will draw more current than the power supply is designed to deliver...."

thanks!
 

blocco a spirale

Joined Jun 18, 2008
1,546
I'm not sure which bits are questions so it would help if you focus on one thing at a time since you have mixed up several ideas into one pot of confusion.

I don't know what you've been reading but with reference to practical audio power amplifiers, for example:
The voltage gain stage will typically have a relatively high input impedance and a high output impedance.
The current amplifier will typically have a relatively high input impedance and a low output impedance.
An amplifiers does, of course, have an output impedance and the author of the linked article doesn't know what he's talking about..
 
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Thread Starter

Spacerat

Joined Aug 3, 2015
19
Ok, let me be clearer:

A voltage amplifier will typically have a relatively high input impedance and a high output impedance. Why does the input impedance have to be high (an open circuit has the highest impedance)? Why would a high impedance be a good choice for the input?

And why the amplifier output impedance should be low (a short has the lowest impedance)?

Can you give me an example using a source having impedance Z_s, a load with Z_load and a voltage amplifier sitting between them?

thanks.
 

blocco a spirale

Joined Jun 18, 2008
1,546
A relatively high input impedance is favourable because it will not load the output of whatever is connected to it. i.e. the source output voltage will be unaffected by the input stage of the amplifier.

A relatively low output impedance is favourable because it can drive the next stage (ideally one of higher impedance) and maintain its output voltage. e.g. an audio power amplifier's output impedance should be considerably lower than that of the loudspeaker that it's designed to drive.

An audio power amplifier typically has several stages of voltage gain at relatively high impedances but the final output of theses stages does not have enough current to drive a loudspeaker so a current amplifier stage(s) is required. If the current amplifier stage had a relatively low input impedance most of that voltage would be lost. Therefore the current amplifier stage has a high input impedance (to minimise loading the voltage amplifier stage) and a low output impedance so that it can deliver the necessary current to maintain that voltage across the loudspeaker.
 
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Lestraveled

Joined May 19, 2014
1,946
OK, lets talk about basic concepts.

Voltage amplifier - The theoretic perfect voltage amp has an infintely high input impedance and a infinitely low output impedance. The reason for the high input impedance is that the input should not draw any current. In other words, the input should not load the input signal. The amplifier output impedance has to be zero because if it were greater than zero then the load impedance would affect the output voltage. It would no longer be a pure voltage amplifier. A pure voltage amplifier's output is not affected by the load impedance.

Does this make sense to you??
 

#12

Joined Nov 30, 2010
18,167
In general, a device has just one single impedance and impedance matching is often required to avoid energy reflection at the connection interface between different elements....
Generally not a problem with audio frequencies. That allows intentional mismatch. High input impedance simply refuses to load down the input signal.
 

grahamed

Joined Jul 23, 2012
100
Hello Forum,
...As stated earlier, the amplifier doesn't have an output impedance....It is the speaker that has the impedance....
Yes, I think I can agree with that.

Of course the amplifier does have output impedance but it is so low (maybe 1/1000x that of the speaker, within the amplifier's linear range -where NFB is effective) that the output current is determined by the unloaded-voltage, the load impedance and Ohm's law only.
 

peter taylor

Joined Apr 1, 2013
106
Oh, this can of worms.

I typically focus on the input signal, and how it connects to the first stage of the amplifier.

With any amplifier, this stage is crucial.

Forget the rest, because this stage will keep you up at night.

My 20 cents
 

peter taylor

Joined Apr 1, 2013
106
Design5.jpg
The complexity of this circuit defies imagination.
This is the first stage.
R1 should be about 10 kΩ to give good base current, good collector current, and good β.
R1 and R2 should be chosen to give a Base Voltage above 0.6 V, whilst not saturating the collector.
R3 should be chosen so that the collector voltage is a smidgen under Vs.
This is slightly more than you would expect your amplified input signal to be.
If R2 = 10 x R1, then your signal will see a load of R1.
And that is it.
I am tired, and don't want to go into AC equivalent circuits, DC coupling, and all things taught at university.
 

Thread Starter

Spacerat

Joined Aug 3, 2015
19
Thanks everyone.

Lestravaled, you mention that the input should not load the input signal.. What does that exactly mean? If the input impedance is other than infinite (or huge), a current will flow into the amplifier. Is that what you mean by "not loading the input signal" on the amplifier, i.e. no current let through? What would happen otherwise?

"...The amplifier output impedance has to be zero because if it were greater than zero then the load impedance would affect the output voltage..."

So the load element is connected to the amplifier output whose impedance needs to be zero. the load is trying to receive an amplified voltage (larger than the voltage the source connected to the amplifier input can provide). I guess the load, whatever its impedance may be, would connect in parallel to the amplifier's output to take advantage of the higher voltage. Your comments is that if in this parallel connection the output impedance was not zero the provided voltage would not be what it is intended to be...

thanks.
 

peter taylor

Joined Apr 1, 2013
106
There is no such thing as zero impedance. Whatever your load, there will always be a finite series resistance.

Otherwise, an infinite current would flow, sucking the Universe in with it.
 

Thread Starter

Spacerat

Joined Aug 3, 2015
19
Sure, zero only in principle. so let's just say very very small....
Still, I am not clear if the load would connect in series or parallel to the amplifier output (and its impedance). I guess in series like an ammeter.
 

Jony130

Joined Feb 17, 2009
5,091
There is no such thing as zero impedance.
Here we are talking about Voltage amplifier and about output resistance/impedance.
And in case of a voltage amplifier the output resistance should be zero. So please stop confusing Thread Starter.
Also you post #10 is very wrong and misleading. For example how can R1 has any effect on transistor beta.

Spacerat please look at this diagram, where I try to show why it is good to have large Rin and small Rout for VCVS (voltage amplifier).
http://forum.allaboutcircuits.com/attachments/0-3-png.28451/
http://forum.allaboutcircuits.com/threads/amplifier-no-load-then-load.25120/#post-153702
 

peter taylor

Joined Apr 1, 2013
106
Because if R1 was 100k, the collector current would reduced to μAmps. If you bother to look at a data sheet, you would see that current gain decreases with current. And if you ever bothered to actually BUILD an amplifier, you would find that collector current and current gain are closely related.
And as far as confusing anybody, one of the things that confused me as a novice, is: if there is no load resistance, then how can this amplifier work?
 

Veracohr

Joined Jan 3, 2011
694
Lestravaled, you mention that the input should not load the input signal.. What does that exactly mean? If the input impedance is other than infinite (or huge), a current will flow into the amplifier. Is that what you mean by "not loading the input signal" on the amplifier, i.e. no current let through? What would happen otherwise?
Consider a simple voltage divider where R1 is the output impedance of the signal source, and R2 is the input impedance of the amplifier. If R1 is too large relative to R2, the output voltage will be smaller than the input. That's what "loading" is; if R2 (amplifier input impedance) is too low and/or R1 (signal output impedance) is too high, the signal voltage seen at the amplifier input will be reduced, and we say that the amplifier circuit is a "heavy load" on the signal source. That's why the "zero output impedance, infinite input impedance" is a useful situation to approximate.

divider.png


What happens to the current when when the voltage is amplified? Does the current decrease to keep the output power equal to the input power?
No. Despite the names "voltage amplifier", "current amplifier", and "power amplifier", they all can amplify power. If you have a 1mV signal from a microphone and an amplifier input impedance of 10kΩ, the input power is .001^2/10,000 = 0.1nW. If you amplify the mic signal to 100mV and drive another stage with 10kΩ input impedance, the output power is .1^2/10,000 = 1μW. You've therefore amplified the power, but since the purpose of the stage was to amplify voltage we call it a voltage amplifier. On the other hand, you can have the same amplifier drive a stage with 100MΩ input impedance, in which case you would have 0.1nW output power, and thus voltage amplification without power amplification. Typically voltage amplifiers have a limited current capability and are thus limited in how much power amplification they can achieve.

"Power amplifier" and "current amplifier" are basically equivalent. The purpose of a power amplifier is to output a voltage identical to the input, but be able to provide much more current (such as to a speaker). Thus it's a current amplifier.
 

Lestraveled

Joined May 19, 2014
1,946
Another way of looking at infinite input impedance is, does the input signal change when it is connected to the input. Does connecting the signal to the input, change the signal in any way.
 

Thread Starter

Spacerat

Joined Aug 3, 2015
19
Thanks everyone. I am going through your various responses.

Veracohr, your voltage divider examples clarifies why the amplifier input impedance must be ideally infinite: if that is the case the signal voltage (which is the potential difference between Signal and ground) faithfully becomes the input to the voltage amplifier. Otherwise, if R2 is not much larger than R1, it will not.

What about the amplifier output impedance being ideally zero? If we use the voltage divider concept, the load would be one of the resistors (load impedance) while the other resistor is zero (i.e. the amplifier output voltage is all across the load and no voltage is across the amplifier output impedance)

So, should we always assume that the input signal connects to the amplifier input in series and the load connects to the amplifier output in series as well?

Also, the type of voltage amplifier here described is VCVS (voltage controlled voltage source). There are also devices that are CCVS (current controlled voltage source) that could have an amplified voltage as output. Are the also called voltage amplifiers?

thanks.
 
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