I have the output of my op amp TLO82 pin one feeding into the base of my 2N3904 via a 1K resistor
the output from the op amp is fine on the op amp side of the resistor but is extremely tiny on the transistor side, I
assume I need to pull something somewhere?

basically how do I interface an op amp with a transistor?

Neil.

 

The precise meaning of "extremely tiny" is very important in this context. You need to provide us with a schematic and a bill of material so we can see what you're trying to do. With the emitter grounded, the base will never be more than 0.7V with respect to ground. If the Vcc of the opamp was +12VDC and it was a rail-to-rail type you might be able to source about 11.3 mA into the base of the transistor. With a wimpy opamp that might be a stretch.

 

basically how do I interface an op amp with a transistor?

It depends on the application. In some cases you don't need a resistor.

Post a schematic.

 

There's no general rule for connecting a transistor to an op amp output.
It depends on what you want the transistor to do.
What's the application?

 

a

... but is extremely tiny on the transistor side...

That's about what I'd expect. As has been noted, "extremely tiny" is subjective and needs details.

You can expect the base to be essentially a short to ground for any voltage above o.7V. So you'll only be getting a few mA through your resistor and you will not be raising the base voltage above 0.7V without a damaging current from collector to emitter in the transistor.

 

how do I interface an op amp with a transistor?

Depends on what the other two transistor pins are connected to. If the transistor is NPN and the emitter is grounded, then the base will be 1 Vf above GND and stuck there.

Schematic?

Paraphrasing Rear Admiral Joshua Painter in "The Hunt For Red October", "An Engineer don't take a dump, son, without a schematic."

ak

 

Yes the tranny is NPN and the emitter is grounded.......that gives me an idea

Neil.

 

Depends on what the other two transistor pins are connected to. If the transistor is NPN and the emitter is grounded, then the base will be 1 Vf above GND and stuck there.

ok I fitted a 1K resistor between the Emitter and ground instead of having the emitter hard wired direct to ground and all works superb,
technically I don't know what I did but your comment about the base being stuck at 1V above ground gave me the idea that the waveform present at the base needed a bit more room to wiggle so thanks Analogue Kid.

This analogue stuff is really fascinating and after I finish my project I am going to teach myself all about transistors, I am really green when it comes to the technical stuff since my math is crap but I love this hobby soooo much

thanks all.

Neil.

 

ok I fitted a 1K resistor between the Emitter and ground instead of having the emitter hard wired direct to ground and all works superb,
technically I don't know what

Technically, you made an emitter follower (or common-collector) amplifier.
It amplifies current, not voltage, allowing you to drive a lower impedance load than the op amp could.

 

When I get time, maybe this weekend, I'll draw up that part of the circuit for conformation.

 

Yes the tranny is NPN

We don't call them trannies. We call them transistors

 

I recommend to add the diode. For instance 1N4148. This for protection from breakdown. Under it is enough greater on absolute value negative source voltages can occur the breakdown. In effect this will degrade the gain (beta) of the transistor

 

Hello,

Tranny makes me think of the old book "resi and transi"
https://openlibrary.org/books/OL20949192M/Resi_Transi_banish_the_mysteries_of_electronics

The formal name to use is bipolair junction transistor or BJT.

Bertus

 

This comment also applies to those who think it is cute to refer to a transformer as a "trafo". It just makes you sound less than literate.

You could be surprised how common that expression is and how frequently is used by your colleagues who deal with them in actual use. I know a couple of them and they never seemed less than literate to me. On the contrary.

BTW, in few days we are discharging two of them, about 120 metric tons weight, 3MVA IIRC, in a terminal not too far from here.

 

Twenty posts without getting anywhere!

That I can find right now is: Op amp TLO82, Transistor 2N3904 and a 1K Resistor.

I would like to see an sketch of the circuit, supply voltage(s), signal levels into the circuit and what is connected to the transistor?

 

I would like to see an sketch of the circuit, supply voltage(s), signal levels into the circuit and what is connected to the transistor?

My question has been dealt with and hence the problem solved, however if I find the time over the weekend I will draw up the relevant part of the circuit and post.

Thanks to those people who helped me here.

Neil.

 

I have the output of my op amp TLO82 pin one feeding into the base of my 2N3904 via a 1K resistor
the output from the op amp is fine on the op amp side of the resistor but is extremely tiny on the transistor side, I
assume I need to pull something somewhere?

basically how do I interface an op amp with a transistor?

Neil.

The easiest arrangement is an emitter follower. The up side is the emitter signal will be the same phase as the op-amp output. The down side is the emitter voltage will be 0.7V lower than the op-amp.

You need to get all the voltage gain from the op-amp because the emitter follower doesn't give any - all the gain is current gain.

In common emitter with no self-compensating emitter resistor; the B/E junction will clamp the voltage at the end of the resistor to 0.7V.

If you're doing linear - you'd be best off if you do the text book self compensating common emitter stage with its own bias resistor network and AC couple it to the op-amp output.

 

ok here is the circuit for the area I was having a problem with, as shown the square wave from the op amp was good on the op amp output but squashed tiny on the base of the tranny (yes I said it unashamedly) then a post from analogue kid got me thinking and I put a 1k in the emitter lead to ground and hey presto.....my square wave is good on the collector

 

I included the circuit and nobody says anything?

 

Not much to say. You solved the problem with an emitter resistor. If you know how a diode works (that whole fixed-forward-voltage-that-doesn't-change-much-with-current thing), then you know why your original findings happened.

BTW, with a resistor in the emitter the transistor now might act as a linear amplifier. The gain of the stage is approx. Rc / Re.

ak