Microphone Amplifier Design Problem

I've just now started to check how my circuit meets the other requirements desired within the design such as the frequency response, input/output impeadance etc...

I tried ranging the frequency from 1kHz to 3kHz and there was no noticeable effect on my amplifiers gain or functionality, so I deemed it capabale of provided the desired frequency response.

Now I've been trying to measure my input and output impeadances but I've been struggling a fair bit.

I originally tried measuring the input and output impedance using the tests described here,

**broken link removed**

But when I was to measure voltages such as V1 and V2 for a known resistance to find the input impeadance, the measurements I found on the meters would vary all over the place, making it hard for me to select a value in which to compute my input impeadance.

I had similiar issues when attempting to preform the approriate tests to determine the output impeadance.

Is it possible to determine these two quantities by simply measuring the resistances at the respected input/output terminals as I have done in the figures posted below?

If this is indeed the case, then I have some more tweaking to do within my circuit to meet the input/output impedance requirements specified in the design. How can I do such a thing without ruining the desired gain that I've established?

It could very well be that I am going about measuring/determining the input/output impedances the wrong way, if that is indeed the case please let me know.

Thanks again!

Your input/output impedance measurement method is wrong. It has to measured as small signal quantity in AC analysis. Input impedance is |Zin| = |Vin/In|. To measure output impedance directly, you have to apply an AC source to the output. Or measure it indirectly by varying the load impedance and determine the gain chance. Actually, both specifications are met with a large margin.

FvM said:

Your input/output impedance measurement method is wrong. It has to measured as small signal quantity in AC analysis. Input impedance is |Zin| = |Vin/In|. To measure output impedance directly, you have to apply an AC source to the output. Or measure it indirectly by varying the load impedance and determine the gain chance. Actually, both specifications are met with a large margin.

Click to expand...

The tests I attempted described in the link of my previous post, are those not valid?

When I attempted to make measurements within my circuit to determine the required voltages for the tests I often found values that had a large variance with left me unsure which value to select.

How do you suggest I go about determining the input/output impedance using the tools in MultiSim?

With wrong method I was referring to your suggestion to use multimeters. The method in the literature is correct, if you understand it as a small signal measurement in simulation. But that's generic SPICE, I can't suggest details for MultiSim.

FvM said:

With wrong method I was referring to your suggestion to use multimeters. The method in the literature is correct, if you understand it as a small signal measurement in simulation. But that's generic SPICE, I can't suggest details for MultiSim.

Click to expand...

Here's my second attempt determining the input impedance using an AC simulation in multisim.

As shown in the figure attached around a frequency of 1kHz we see an input impedance like the following,

\[Z_{in} = 49.9k \angle 0.006^{o} \Omega\]

Does that correspond with values you were finding FvM?

If so, I will deduce my output impedance in a similar fashion.

The value sounds reasonable. I modified R2 and got a slightly larger value. However, when measuring the input impedance at the real input, C1 will increase the magnitude at low frequencies.

FvM said:

The value sounds reasonable. I modified R2 and got a slightly larger value. However, when measuring the input impedance at the real input, C1 will increase the magnitude at low frequencies.

Click to expand...

That's good to hear!

Here's what I've found for the output impedance,

\[Z_{out} = 14.172 \angle 0.08737^{o} \Omega\]

Does this coincide with what you had found for the output impedance?

Yes, very similar.

Towing Orange County & Long Beach

I am attempting to design a amplifier for microphone and for that i am little bit flustered how to start to design.

Hello again everyone.

It seems that as though there are a couple of things I had overlooked in the design of my amplifier.

Firstly, on the second page of the data sheet for the microphone (See figure attached, squared in red) there is a resistor that must be hooked to the battery in order to power the circuit for the microphone. This was not included in the circuit I've been working with thus far.

Secondly I did not account for the output impedance of the microphone. (See figure attached, squared in red)

I decided to add these things into my circuit to see how it was going to effect it's preformance.

Before doing so, I had some confusion about whether or not the output impedance of the microphone is in series, or parallel with the small ac voltage obtain from the microphone circuit. For this reason, I divided things into 2 cases, one where it was in series and one where it was on parallel. (Case 1 = Parallel, Case 2 = Series)

See figures attached for the 2 cases described.

As you can see in Case 1, my circuit holds its gain and other various characteristics and works fine.

In Case 2, my circuit loses it's gain, as the voltage at the input is much larger than before. (The order of mV not uV)

First question that needs to be adressed is which case is indeed correct based on the datasheet from the microphone? Case 1 or Case 2?

The second question relates to the answer of the first question.

If it is Case 1: is this output impeadance going to ruin my input impedance that I've established previously? If so, how do I go about fixing that?

If it is Case 2: How do I go about correcting the issues I'm having with my gain? Is there any effect on my input impedance? And if so, how do I correct that?

Thanks again!

1. The requirement for microphone power supply is missing in the amplifier specifications yet.
2. The microphone ouput resistance has to be modelled correctly as a voltage source with series resistance. Alternatively, you can use a current source with parallel resistance.
3. I can't understand your results about "ruining gain". They contradict the previously determined amplifier input impedance of about 50 kohm, which results in a voltage loss of just 1%. Something is obviously wrong in your measurement setup.
4. The low level output waveform shows some cross-over distortions, caused by too low bias current of the output stage. I already mentioned it in post #17. I would keep this point for later circuit optimization.

jegues said:

... on the second page of the data sheet for the microphone (See figure attached, squared in red) there is a resistor that must be hooked to the battery in order to power the circuit for the microphone. This was not included in the circuit I've been working with thus far.

Secondly I did not account for the output impedance of the microphone.

Click to expand...

I'd suggest to use the following equivalent circuit schematic for your electret micro:

Cmic is just a virtual capacitor inside the mike-amp (actually not existent, just necessary for the equivalent circuit). Use a (very) large value for it, ≫ C1 . Your C1, however, may be much smaller: 1..10µF is sufficient.

The power supply resistor R7 should supply 0.5mA @ 3V , so 12kΩ.