I'm working on a fairly high gain circuit for listening to VLF whistlers, etc. In effect it's just an audio amp with a hi-Z input impedance FET (BF256) attached to a metre or so of wire and a bipolar transistor (BC109) feeding an LM386. (There are probably better amps than the LM386, but unfortunately I'm stuck with it.)

Question: Use the inverting or non-inverting LM386 input? Nearly all amp circuits use the non-inverting input, but isn't that inviting instability problems or is there something I'm missing? Obviously layout is important to reduce stability issues, but does the choice of LM386 input depend upon how many stages precede it, so that the output is inverted related to the input?

(I did use the forum search facility, but didn't see anything that obviously answered this question.)

Thank you.

Barbara

 

The LM386 is an audio amp, not an operational amplifier, so it does not have the instability problems associated with op-amps. Nevertheless, especially in its high-gain configuration, it does suffer from internal noise & external interference problems but these can be suppressed through copious use of decoupling capacitors and frequency response shaping. The website http://stephan.win31.de/music.htm has a LM386 section explaining, "In its usual application, the NSC LM386 pretty much employs a single-ended input stage. While it does essentially have a differential amplifier, one half of it is merely used as a ground level shifter and for setting output stage offset." So typically one would decide whether the amplified output should be in-phase with the input signal, or not, then connect the signal to the appropriate input and ground the other input.

 

The LM386 is an audio amp, not an operational amplifier, so it does not have the instability problems associated with op-amps. Nevertheless, especially in its high-gain configuration, it does suffer from internal noise & external interference problems but these can be suppressed through copious use of decoupling capacitors and frequency response shaping. The website http://stephan.win31.de/music.htm has a LM386 section explaining, "In its usual application, the NSC LM386 pretty much employs a single-ended input stage. While it does essentially have a differential amplifier, one half of it is merely used as a ground level shifter and for setting output stage offset." So typically one would decide whether the amplified output should be in-phase with the input signal, or not, then connect the signal to the appropriate input and ground the other input.

Thanks for the link. Very helpful stuff there - not just LM386 info!

 

Use the inverting or non-inverting LM386 input?

with the lm386, you don't have a choice.