Power Amplifier vs Voltage Amplifier

[aryajur]

My question is a very basic one. Simply putting why do we need a power amplifier? Putting this in another way -- if we are receiving a signal say from an antenna, which has a output impedance say 50 ohms. So why would it be better to have the stage connected to this antenna have an input impedance = 50 ohm rather than have an impedance = infinite i.e. why would we want maximum power transfer, rather than maximum voltage transfer.
I am confused because the signal that the antenna is receiving, if we know the voltage waveform of the signal then that signal can be reproduced easily, isn't it? So why not have a voltage amplifier connected to the antenna???

 

[flatulent]

The voltage-power question on amplifiers deals with the load impedance the work into and the efficiency desired and the signal level.

The antenna question is different. The antenna-transmission line impedance should be transformed into the source impedance that has the lowest noise figure for the amplifier.

The antenna amplifier is always small signal so the efficiency of the amplifier will always be low and the output will always be low.

 

[metal]

Hello

Another thing is that mismatching in the antenna and input circuit causes low SNR, also you can think of power transfer issue as a big circle in which the voltage issue is located.

How can you achieve the best voltage available, even if its weak and we know how it looks like if you are losing power in mimatching impedances.

Regards

 

[aryajur]

Thanks for the replies. I am trying to understand it better. One confusion is like if we make a Common Source amplifier with a MOSFET, it is like a very good voltage amplifier. Or if we make a Common Emitter BJT amplifier, it is a very good current amplifier.
So how would we use this basic amplifying device to achieve a good SNR? Like we can make the input impedance equal to the source impedance by adding a parrallel resistor, but wouldn't that dissipate the power on that resistor element rather than putting it through our main amplifying component??

 

[nand_gates]

Hi,
Generally while dealing with RF we dont talk about voltage or current amplification,
we talk about power amplification. This has to do with the fact that at RF circuit theorms
fails and we need to use field theorms i.e. maxwells equations for analysis! So in RF
circuits terms like voltage or current amplification has no meaning.
Yes you can connect ur antenna to voltage amplifier but before that you need to
do impedance matchiing means you use a step-up transformer which will step down
current and step up voltage. maintaining p=i*v constant!
Same thing is applicable to current ampifier!

Hope these hints help you undestand more....

 

[aryajur]

Problem revisited...
It made sense to have a matching if we want to drive something like a motor from a power source, in that case we need the maximum power possible from the power source.
In case of an RF system, we need power matching (say from antenna to the input of the amplifier) because we need to have the maximum SNR at the input. We take the SNR as the ratio of powers of the Signal and the noise. Suppose we had somehow characterized the noise of the amplifier as a voltage, then would we be finding SNR as the ratio of the voltages????
In that case does that mean we would just be worried about the maximum voltage transfer?? It seems strange that our choice of analysis and design should depend upon how we characterize noise.

Please clarify anyone...

 

[jordan76]

Hi,
Sure, our choice of analysis and design should NOT depend upon how we characterize noise!
Recall that SNR is actually the ratio of signal power over noise power, whether we characterize noise as voltage or as current is determined by the convenience of our analysis though both are equivalent eventually.

Hope that will throw some light on your question.

regards,
jordan76

 

[aryajur]

Still confused, the basic part is still unanswered, well then why do we care so much about the maximum power transfer from the antenna to the amplifier, why not just the voltage signal??????

 

[samcheetah]

aryajur this topic also confused me alot in the beginning. the maximum power transfer theorm states that maximum power will be transferred from the source to the load when the impedance of the source is equal to the impedance of the load. and in that case the maximum power transferred will be 50%. that means that half will be wasted in the source impedance and half will be used by the load. now this is the maximum power that can be delivered from a source to a load. and this is the main reason for impedance (or conjugate) matching in power amplifiers.

power and voltage are two different things. power is V times I so it is the voltage mutliplied by some factor (I). in a power amplifier we need both current and voltage amplification. while in a voltage amplifier we do not care about the power. an antenna needs energy to operate so the signal should have alot of power to propagate a signal

 

[aryajur]

Hello Samcheetah,
Thanks for a reply, but still, its the same thing, I understand that we need huge power to TRANSMIT so it would be best to have the maximum power transfer to the antenna that is transmitting our signal.
But the case that confuses me is why do we need maximum power transfer, if we are receiving the signal from an antenna. In that case our information is in the shape of the signal so we should only be concerned about replicating or amplifying the signal shape not the power !
So why are we so concerned with preserving the signal power on the receiving end?

 

[samcheetah]

the theory of reciprocity applies here. the properties of the transmitting end and the recieving end should be same with only one difference. that is that at the transmitting end the soure of the signal is the signal generator and at the reciever end the soure is the EM wave. so at the recieving end the recieving antenna can be viewed as being a voltage source with its internal impedance (antenna radiation and loss resistance) and the load is the reciever circuitry.

i hope that helps

 

[aryajur]

Thats my point, if we view the antenna as a voltage source why don't we just care about voltage amplification of the signal from the antenna, why even bother for maximum power transfer from the antenna??

 

[samcheetah]

okay i now get the problem. well the problem lies in the convention we use for naming supplies. whether it is a current source or voltage source it supplies power. the only difference between the two is that a voltage source gives a fixed voltage and the current varies according to the load and a current source gives a fixed current and the voltage varies according to the load. but both types of sources supply power.

when you say that this black box is a voltage source then it means that it is a source of power. voltage is nothing but the difference of potential between two points. voltage is not something that can be supplied to something. the thing that is supplied is the power.

so when i said that the antenna can be viewed as a voltage source i meant that it was a power source and which behaved like a voltage source (fixed V and variable I). but the point is that the thing that is being supplied is energy (or power which is the time rate of energy). the reciever antenna can also be viewed as a current source in parallel with a resistor by source transformation theorem. so it doesnt matter whether it is a current source or a voltage source the main thing is "power"

i hope that helps

 

[aryajur]

Thanks that narrows it down a little but see, the thing still remaining in my mind is that, the information that the signal (being received by the antenna)contains is encoded in the voltage of the signal and not the power, isn't it? So we could very well forget about the power and just treat the antenna as the voltage source since it directly corresponds to the information and then amplify the voltage, i.e. amplify our information.
I am sorry for just dragging this on, but I really like to get this cleared up. So please tell me where am I going wrong now??

 

[samcheetah]

as i told you that the voltage is nothing but the difference of potential between two points. and even if we consider the antenna as a source of voltage it would be giving us power. because if we have a voltage between two points but there is no flow of charge then that voltage cant do work. and for it to do work there must be a flow of charge (current) and voltage times the current is the power.

when you say that the information is encoded in voltage you actually refer to the waveform that you see on an oscilloscope. but actually the information is transmitted from one place to another is by doing work the rate of doing work is power.

so at the transmitter end we encode the information and send it as an EM wave which basically is energy. now at the reciever end we need to capture all that energy that was transmitted in the first place. thats why we use impedance matching so that maximum power (or energy) is transferred from the captured wave to the reciever circuitry.

as you have said in a previous post that you understand that why maximum power transfer is needed at the transmitter end. well thats good because the same theory applies at the reciever end. the only difference between the transmitter end and the reciever end is that the direction of flow of the signal is opposite. so as you know that we try to transmit maximum power at the transmitter, in the same way we try to recieve maximum power at the reciever.

i hope that helps