Monday, January 7, 2019

a pre amp build and test

If you have followed my blog you will realize I like circuits that are easy on the battery. So I may sacrifice a little performance to conserve battery. With this in mind look at the sims with different load resistors. While low Z circuits require higher currents we can sometimes use lower current and still get reasonable gain.
 This is the circuit as designed. 8mv output but draws 7ma.
with a 2.2k load we get 6mv with 3.4 ma battery load.
with 3k load we get 5.4mv out with 2.5ma drain on the battery.
With a 10k load we get 2mv at less than 1 ma drain. If 7ma drain is good with you the first circuit will be good for you. I like the 3k at 2.5ma draw. I have some 3k in reach so that is what I used. Let's look at the circuit.
The yellow probe is on the input and the green probe is on the output. Both traces are on the same range.
I increased the signal so the yellow trace would display a curve. I had to increase the range for the green trace. It is giving a good boost to the signal. The test was at 4Mhz. I sweep the signal to about 10Mhz and down to 100Khz. As would be expected the signal was stronger at the lower frequencies. It is impressive for so simple a circuit. 1 transistor, 2 capacitors and 3 resistors. Using surface mount components it can be made very small. I have some 1/2" tubing to try and mount an amp in. It will be easy enough to mount on a small piece of board or glass. A microscope slide could work well. Mounting a connector will be the challenge. The tubing is the same size as a BNC connector. It could be swagged to make it work. An altoid box may be better. with a divider it could hold two amps.

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