JFET maximum gain?

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Long story: I have a design that has a 3.5mm audio jack and was meant to interface with dynamic microphones. The requirements changed after the board was designed and has to work with both electret microphones and dynamic microphones. The input signal is coming in from the 3.5 mm jack, to an active filter with gain, and to an audio codec. Unfortunately the audio codec does not have software control of the input microphone level, so I am left to do this in hardware. The problem is that the dynamic microphones are 11-12 mV peak to peak and the electret microphones are 300-500 mV peak to peak. Also the dynamic microphones do not need phantom power whereas the electret need"phantom power. There's also no microcontroller on board.

My solution to this problem was that I was going to put a 2k pullup resistor to the 5V rail and connect that to the tip/ring of the 3.5 mm jack. This will work well for the phantom powered electret and find an optimal gain for the electret microphone with the active filter.

With the dynamic microphone, I was going to have a special cable harness built (which they said was ok) and add a JFET pre amplifier inside the cable, with the dynamic microphone connected to the gate of the amplifier. I was hoping to get a gain equal to the electret microphone (a gain of about 40) so that I could get away with using the one active filter on board.

When I tried adding a JFET, I went out to Radio Shack and purchased the only one they had, the MPF102.

I used this exact schematic as a test, but was only able to achieve a gain of about 1.2 to 1.4 when adjusting the source and drain resistors from the schematic after trying the exact values in the schematic. I did use a 12V supply even though I only have 5V available on board, I figured I could order a JFET off Digikey once I prove that this works.

Short story: Is there any way to get a gain of about 40 from a JFET? Is there a more scientific way to self bias the JFET or is it sort of guess and check (I browsed a bunch of non-engineer tutorials I think). Any help on how to get some nice gain out of this JFET from a dynamic microphone would be greatly appreciated.

Thanks!

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For higher gain the 4.7uF by pass cap is not optional

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I used a 10 uF bypass cap.

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1. You've just 5V available: that *is* an issue with JFET's.
2. If you look at the datasheet of the MPF102, you'll notice the wide spread in the transfer characteristics.

To get more gain out of the circuit, you need to increase the drain resistor. But that limits also the sourcecurrent and therefor the negative bias of the gate. Catch22 ....

Using a bipolar transistor however, will give you the required gain. And at 5V operation :)

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Dynamic mics are low impedance. My experience is that FETs are useful when bipolars don't work* and that is not very often. A bipolar is the best choice here, the input impedance is low and so you can get lots of gain, often using a common base configuration.

You may consider a cascode configuration with the one transistor at the mic and one at the filter/amp.

Use LTSpice to model your circuits.

*Why bipolars and not FET's
1) more gain, less noise with source impedances below 100K
2) More reliable when voltages are low (FET's have a very wide spread -compared to a bipolar- in critical parameters like Vgs and Idss).
3) Lower power, they work right into saturation
4) Fet type switches are sometimes used without series gate resistors - bad idea, they can oscillate, always add a few hundred ohms.
5) Far more suppliers for bipolar parts
6) MOSFETS input leakage is bad over temperature, you lose the impedance advantage at high temperature

GK

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Thanks for the responses. We actually decided to build a small board with a microcontroller on it and a variable gain amplifier. Now if it's an AVR or PIC, that's another story... haha, I like the AVR's but the other guys at work like PIC's.

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A suitable op amp is another option. They are available in tiny packages.

Leon Heller G1HSM

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That circuit looks like the back of the Radio Shack pack that the MPF102 comes in. I used that curcuit for a guitar preamp back about '70 and it sounded real bluesy. Just make the drain r a little bigger and the source r a little smaller, and see if the gain goes up enough.

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