Impedance mismatch issue in receivers

Thread Starter

naumankalia

Joined Feb 19, 2016
42
Hi all

I have an acoustic piezoelectric sensor whose output impedance should be ideally 50 ohm and a preamplifier circuit specifically designed for this sensor having input impedance of 50 Ohm. The piezoelectric sensor output voltage range is from uVrms to few mVrm. These low voltage signals are then amplified by preamplifier.
My query is that if there is a mismatch b/w output impedance of sensor and input impedance of preamplifier, what will be the side effects? In case of Transmission chain, it is obvious that such mismatch (e.g. b/w Power Amplifier and Transducer) causes power losses, however, what will happen in case of receiver chain when very low voltage signals are present ?
 

KeithWalker

Joined Jul 10, 2017
2,398
Hi all

I have an acoustic piezoelectric sensor whose output impedance should be ideally 50 ohm and a preamplifier circuit specifically designed for this sensor having input impedance of 50 Ohm. The piezoelectric sensor output voltage range is from uVrms to few mVrm. These low voltage signals are then amplified by preamplifier.
My query is that if there is a mismatch b/w output impedance of sensor and input impedance of preamplifier, what will be the side effects? In case of Transmission chain, it is obvious that such mismatch (e.g. b/w Power Amplifier and Transducer) causes power losses, however, what will happen in case of receiver chain when very low voltage signals are present ?
The answer depends on the design of the amplifier. The output of an amplifier with a very high input impedance depends the input voltage, not the power. Its input would be twice the voltage that would appear on a matched 50 ohm input.
 

MrChips

Joined Oct 2, 2009
26,462
The purpose of matching the 50Ω load impedance to the 50Ω source impedance is for matching to a cable with 50Ω characteristic impedance. This will minimize signal reflection with fast rising and falling edges.

The signal at the receiving end will be half the amplitude of the transmitted signal.
 

crutschow

Joined Mar 14, 2008
30,075
Matching input and output impedances are usually only required for RF signals.
For low frequencies, where signal reflection on the signal line would not be a problem then, for maximum signal transfer, you want the preamp to have a much higher input impedance than the sensors output impedance.
 

Thread Starter

naumankalia

Joined Feb 19, 2016
42
Matching input and output impedances are usually only required for RF signals.
Frequency of signal in my case is around 110 KHz.
For low frequencies, where signal reflection on the signal line would not be a problem then, for maximum signal transfer, you want the preamp to have a much higher input impedance than the sensors output impedance.
Yes, you are right but the problem is that both sensor and preamplifier have been developed and presently it is not feasible to modify the design. I just need to know that if e.g. a sensor having output impedance of say 67 ohm is connected with preamplifier having input impedance of 50 ohm, what will be the consequences in terms of signal loss, noise etc?
 

crutschow

Joined Mar 14, 2008
30,075
ust need to know that if e.g. a sensor having output impedance of say 67 ohm is connected with preamplifier having input impedance of 50 ohm, what will be the consequences in terms of signal loss, noise etc?
The is a signal loss proportional to the difference in impedance.
It's just simple Ohms law for a two resistor voltage divider.

For a matched load, the signal would be 50% of the open circuit value.
For a 67Ω load, the signal would be 50 / (67+50) = 42.7% of the open circuit value.

The noise of the signal would be unchanged, but the signal to amplifier noise ratio would be worse.

Can't you just remove the 50Ω load of the preamplifier?
 
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