How does this circuit work? Is it a pre-amp?

U6A is a current input amplifier, so the output voltage is -I_in * R13.

C5 is a DC-blocker not a filter; but it needs to be much larger than C7 to ensure most of the signal current flows into the amplifier summing node.

The unnecessary voltage follower U7A has a capacitor from its output to ground, which is a bit odd.

C6 and C7 are decoupling capacitors, as you spotted.

Thanks, Andrew! How would I be able to tell in the future if an opamp is a current amplifier or not? On this site,

, I don't spot the indication right away.

So the value of C5 is not determined by any frequency at all?

Actually, this is the rest of the circuit:

(highlighted in the green box). I'm still not sure what U7A does.. I figured it's independent from the other part (highlighted in green).

Thanks!

It's rubbish. Probably 'engineered' by a monkey on an off day.

Graham

By the way, is a current input amplifier synonymous to a current feedback amplifier? If so, Opamps for Everyone has the following statement: "In general, VFAs (voltage feedback amplifiers) are used for precision and general purpose applications, while CFAs (current feedback amplifiers) are restricted to high frequency applications above 100 MHz."

It's shockingly incompetent..

Whoever put this lot together need to be shot.

Graham

Current feedback is something else entirely.

Graham

Hi Graham, I'm still learning about circuit configurations, so I really could not tell. Thanks!

Believe it or not, it was another classmate's instructor.. I just copied the drawing on the computer because it was printed on paper. It is supposed to be an FM transmitter circuit, and he asked me for help in figuring it out....so I came here to get some answers.

The paper also says: "L2 is 112nH (this tunes to the middle of the FM band, 98 MHz), with C13 at its center value 33pF. L2 is 5 turns of 22 swg enameled copper wire close-wound on a 5mm diameter former. You can also have a fixed 33pF capacitor instead of the variable C13 and have L2 as an adjustable molded coil."

... btw, what's a "former"?

Thanks, Graham! So how do I determine if the LM158 or any other ICs is a current amplifier?

The LM158 is not intrinsically a current amp. The circuit configuration in which it is used is a current amp. Current flowing into the summing node has nowhere else to go but through R13.

It's fairly non-critical. It needs to be large enough to NOT be a high pass filter at audio frequencies.

The circuit is an FM transmitter. U7A is an isolating amplifier. The 1nF capacitors are there to prevent RF feedback causing instability.

Thanks, Andrew! I understood most of what you said except for (regarding C5):

If C5 does not become a high pass at audio frequencies, then how is the audio source getting thru past the opamp stages? I had the impression that C5 would short as the frequency gets higher. Isn't that what a DC block does?

I should have said the configuration is a current INPUT amp, or trans-resistance amp i.e. current input, voltage output.

It works like this:

The voltage at U6A pin 2 is held at VCC/2 by negative feedback. It never changes. The capacitance microphone produces a small AC speech current which flows through C5. Most of this current flows on through R13. Only a vanishingly small fraction of the current enters pin 2.

The output voltage is therefore VCC/2 + I*R13

Nothing surprises me any more.

Uh ? It is ?

Hmm.

A former is something on which you can form a coil e.g.a piece of plastic tubing.

Grahsm

You're confusing 'current amplifier' with 'current feedback amplifier'. Not the same thing at all.

Graham

What kind of modulation ?

Graham

Correct. I meant that C5 needs to be large so it doesn't attenuate the lower audio frequencies. Strictly speaking, it is a high pass filter; but to call it one suggests that only high frequencies pass through; when in fact the cutoff frequency is so low that all audio frequencies can pass. Only sub-audio and DC are blocked.

I'm totally mystified by this circuit. If it really is an FM transmiter, then where is the carrier tuning? I have seen FM transmitters that used a condenser (capacitor) mic as the modulator for the tuning circuit, but that doesn't seem to be the case here. And if it is a transmitter, wouldn't C4 be trying to filter out the whole signal? (Assuming that's supposed to be the output, since there is no antenna connection marked,)

BTW, if that really *is* supposed to be a condenser mic, good luck finding one at a reasonable price: These are typically only used as precision reference mics, at US$ 1000 and up. The cheap little electret mics that we see everywhere use the same underlying technology (sort of), but they have a permanently charged backing plate (the electret) and, unfortunately for tuning uses, an FET preamp that isolates the capacitance of the mic from the output and turns it into a voltage change.

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

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