Thread: Impedance Output and Impedance Input

What's the relationship between output impedance and input impedance?

In one scenario I have a voltage buffer with an output impedance, which in some way should relate to the input impedance of a bandpass filter.

In another scenario, I can calculate the output impedance of a low pass filter and the input impedance of a high pass filter, in a band pass filter. But how do they relate to each other?

Should I be performing some be adding/multiplying/etc the output impedance with the input impedance or include the output in the equation for impedance?

It's said that an output impedance of a voltage buffer is fed into the bandpass filter.

If the the impedance values of the connections don't match you get a reflection of the wave and it reduces signal strength as well as introducing some noise.
At least that is what happens with antennas.
Not sure about voltage buffers and highpass filters because I don't know the load characteristics of them.

but Wikipedia has a nice article on impedance matching that might help.

Originally Posted by dan hunter

If the the impedance values of the connections don't match you get a reflection of the wave and it reduces signal strength as well as introducing some noise.
At least that is what happens with antennas.
Not sure about voltage buffers and highpass filters because I don't know the load characteristics of them.

but Wikipedia has a nice article on impedance matching that might help.

But resistor values in a bandpass filter must be different.
So the impedance output of a low pass filter would be different from the high pass filter.

Are you saying that I should try to build a bandpass filter where the impedance output of the bandpass filter's low pass filter should match the impedance input of the high pass filter?

Originally Posted by Mechatron

What's the relationship between output impedance and input impedance?

When output impedance equals input impedance power transfer is maximized.

In one scenario I have a voltage buffer with an output impedance, which in some way should relate to the input impedance of a bandpass filter.
In another scenario, I can calculate the output impedance of a low pass filter and the input impedance of a high pass filter, in a band pass filter. But how do they relate to each other?

Not sure of what you are asking. The output impedance of one stage becomes part of the circuit of the next stage. Thus if you change the output impedance of an op amp, it will change the frequency response and/or gain of the next stage if it is expecting a given impedance.

Should I be performing some be adding/multiplying/etc the output impedance with the input impedance or include the output in the equation for impedance?

Just analyze the circuit using the actual output impedance.

Originally Posted by dan hunter

If the the impedance values of the connections don't match you get a reflection of the wave and it reduces signal strength as well as introducing some noise.
At least that is what happens with antennas.
Not sure about voltage buffers and highpass filters because I don't know the load characteristics of them.

but Wikipedia has a nice article on impedance matching that might help.

I can tell you that there will be no impedance at all out of the voltage buffer.
So it can't possibly match the input impedance of the bandpass filter, unless that's also zero.

Originally Posted by Mechatron

I can tell you that there will be no impedance at all out of the voltage buffer.
So it can't possibly match the input impedance of the bandpass filter, unless that's also zero.

All buffers have some output impedance (i.e. they are not perfect.) Often it is assumed to be close to zero, which works as long as you add another resistance after the output to change the impedance to what you want. For example, if you have an op amp with a 5 ohm output and you add a 220 ohm resistor afterwards, the impedance is effectively 220 ohms (which you would then use in your analysis.)

Originally Posted by billvon

Originally Posted by Mechatron

I can tell you that there will be no impedance at all out of the voltage buffer.
So it can't possibly match the input impedance of the bandpass filter, unless that's also zero.

All buffers have some output impedance (i.e. they are not perfect.) Often it is assumed to be close to zero, which works as long as you add another resistance after the output to change the impedance to what you want. For example, if you have an op amp with a 5 ohm output and you add a 220 ohm resistor afterwards, the impedance is effectively 220 ohms (which you would then use in your analysis.)

It is my understanding that input and output impedance requirements depend on whether one is dealing with voltage, current, or power. For example, matching the output impedance to the input impedance of the following stage is used to maximise the power transfer between the two stages. For voltage applications, the ideal is infinite input impedance and zero output impedance, whereas for current applications, the opposite is true (zero input impedance and infinite output impedance).

A really easy way to visualise what is going on is to think of the output impedance of the previous stage and the input impedance of the next stage as two resistors connected in series, with the output voltage of the previous stage applied to the 'top' of the network and the 'bottom' of the network connected to earth, (I would draw a diagram if I knew how to on here) The best I can do is;

|
[] R1 = previous stage output impedance
|
[] R2 = next stage input impedance
|


Knowing the output voltage of the previous stage, you can now work how how much current will flow through the two 'resistors' and therefore what voltage will appear across the input impedance of the next stage, ie across R2

Frequency dependant networks such as filters etc. will often have input and output impedances that vary with frequency. So unless they are very sophisticated (ie, complex and expensive), you will need some sort of buffers on the input and outputs. Broadcasters in the UK used to match their audio circuits to have 600 ohm impedance on all outputs and inputs. However, they changed to about 75 ohm output impedance and 50k ohm input impedance*

All this applies to low frequencies. As Dan said, when you get up to radio frequencies, impedances become much more critical, leading to reflection problems if they are mismatched.


*from memory, this was a long time ago for me.

You're welcome. Don't mention it.

OB