Understanding Input and Output Resistances of an Amplifier with Feedback"

In summary: Just apply a small DC voltage and measure the resulting DC current. This will give you the input and output resistances (or impedances) of the amplifier, which can then be used to calculate the feedback gain (beta). In summary, to measure the input and output resistances of an amplifier with feedback, you can use a voltage source and measure the resulting current for input resistance, and a load resistance with a current meter for output resistance. For more accurate measurements, a network analyzer can be used to measure S parameters which can then be converted to impedance. The feedback gain (beta) can also be calculated using these measurements. At low frequencies, a DC measurement can be used instead of a network analyzer.
  • #1
engcon
12
0
Hi,

Suppose I have an amplifier with feedback, how would I measure the input and output resistances (using instruments)?

If the input of the amplifier is a voltage source, the input resistance would simply be V(signal) / I(signal).
Now if the output of the amplifier is a current, would I just need to hook a load resistance and measure the voltage across and then divide by the output current?

Lastly, is it possible to measure beta (the feedback gain), or does that have to be done mathematically?

Many Thanks
 
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  • #2
Yes, that's how you do it. R = V/I for input and output. For amplifiers, it is more useful to measure input and output impedance Z = V/I, as there is that imaginary component.

It is impractical to measure current at higher frequencies, therefore this method is seldom used. The preferred method is using a network analyzer, which measures S parameters very accurately. The advantage of that is S parameters can be converted to any type of parameters including the impedance.

If you are working on simple low frequency BJT amp, then input and output resistances can be easily calculated.

I'm not sure which beta are you referring to? Transistor current gain? or negative feedback as some books use beta? In either case both can be measured and calculated.
 
  • #3
Hi, thanks for your reply!

waht said:
I'm not sure which beta are you referring to? Transistor current gain? or negative feedback as some books use beta? In either case both can be measured and calculated.

By beta I was referring to the negative feedback, not the transistor current gain (beta or hfe).

(The amplifiers in question will have to be tested with a maximum frequency of 10kHz)
 
  • #4
At 10 kHz, you probably don't have to use a network analyzer. Since the frequency is so low, you can get away with a DC measurement.
 

What is an amplifier with feedback?

An amplifier with feedback is an electronic circuit that uses a portion of the output signal to regulate the input signal, resulting in an amplified output. This feedback mechanism allows for more precise control and stability in the amplification process.

What is the purpose of feedback in an amplifier?

The purpose of feedback in an amplifier is to regulate and stabilize the amplification process. It also helps to reduce distortion and improve the overall performance of the amplifier.

What are the different types of feedback in an amplifier?

There are two main types of feedback in an amplifier: positive feedback and negative feedback. Positive feedback amplifies the input signal, while negative feedback reduces the input signal. Negative feedback is the most commonly used type in amplifiers.

What are the advantages of using feedback in an amplifier?

The use of feedback in an amplifier has several advantages, including increased stability, reduced distortion, and improved linearity. It also allows for greater control over the amplification process and can help to minimize external noise.

What are the potential drawbacks of using feedback in an amplifier?

While feedback can improve the performance of an amplifier, it can also introduce some potential drawbacks. These include increased complexity, potential for instability, and the possibility of introducing unwanted noise or oscillations.

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