EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: dusan on September 30, 2023, 07:20:52 am
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I'm making a piezo preamp and the board is too small and I have trouble placing as large output cap as I would like on the board. I'm using approx. 1-1.5m cable, is it the same if I place output cap at the receiving end?
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Shift the emitter resistor to the other end of the line. No worries there.
How big is your cap? You can get a 16V 10u in an 0805 size, about the same size as an smt npn.
eg CL21A106KOQNFNE from Mouser
Is your amplifier input impedance so low that it needs a 4.7u coupling cap?
It needs to be >> 20k to avoid loading the follower.
Consider using a source follower instead.
If you have room for a transistor, you've got room for an SC70 opamp eg LM321LV
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That high-capacitance SMT ceramic is X5R dielectric, so if you use it you want the input impedance of the next stage to be large enough that there will be negligible AC voltage across the capacitor at low frequencies to minimize non-linearity problems (hence the need for a high capacitance value). Note that Mouser does not stock that particular part number (factory order minimum 200 k parts !).
The emitter resistor and coupling capacitor can go at the receiving end of the line, as pointed out by Terry Bites.
If you have room at the receiving end, consider using a film capacitor instead.
Another reason to use an unexpectedly large coupling capacitor is to reduce the noise of the next stage due to its equivalent input current noise generator (especially if the next stage is a BJT) due to the total impedance seen by that stage's input at low frequencies.
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Is your amplifier input impedance so low that it needs a 4.7u coupling cap?
Input impedance is 1700 ohm (soundcard)
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I think I'll buy 10uF in 0805