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[SOLVED] Another question about op amp biasing

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obrien135

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what exactly do you mean by weird? that is an ac coupled op amp circuit, so as long as the - and + inputs are the same voltage (around Vss/2) it is correct.
 

You might be messing up the bias just by probing on the circuit. Remember that your meter has some internal resistance (100k to 1M, typically), so redraw/resim the circuit with an additional shunt resistance on the node you are trying to probe, and you might get similar results.

Have you tried running an AC sweep through it and seeing if the gain profile is correct?
 

Well, Vss is 13.8 VDC. I get a little over 12VDC on the output pin. on the + and - inputs I get a little over 11 VDC. I made sure that the voltage divider for the + input was connected to ground and that all of the connections were made. I checked for shorts. Everything looked normal. It looked as though the 100pF capacitor from the feeedback loop to ground was charging up and dragging the input pins up with it. Is that because I used 100K's on the voltage divider and 1K's on the feedback loop? PLease see the attached diagram. I didn't use an offset null circuit with this design, but I didn't have this problem until I put in the 100 pF capacitor. Maybe I shoud use lower than 100K's in there, or instead of the 100pF cap use a zener diode or a low resistance voltage divider to hold that one end of the 1K resister at 1/2 Vss. Getting the parts to keep making modifications is an issue due to financial constraints and the low resistance voltage divider would be the cheapest and easiest method, if there's a chance it would work. I don't know why it wasn't shown that way on the website.

Maybe the op amp is shot.

I don't have any simulation software. I've been prototyping. It's a pain that way.
 

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You do realize you chose the "unstable" circuit. Also, 100 pf is way too small.

You should also check to see if it is oscillating.
 

I see. So by adding the resister between the divider and the + input (and the coupling cap output) it will be more stable? And the cap also? It isn't oscillating. The input is connected to a colpitts oscillator and I'm not seeing any signs of a signal from that either. From my understanding Xc of the 100pF should be << than the 1kohm in series with it. It's reactance at 10MHz is about 160 ohms. Isn't that small enough?

---------- Post added at 11:59 ---------- Previous post was at 11:54 ----------

Wouldn't the capacitors be expected to charge up that way? with that cap beteern the 1k and ground?
 

Yeah, make the 100 pf more like .1 uf and it will work better. Also, you need a different op amp, as the one you chose only has around 1.5 MHz large signal bandwidth.
 
Yes I am going to swap in an LM7171. Does anyone know if those biasing voltages are to be expected with that cap in there? Its seems to me they would be. Which makes me wonder how that type of circuit would ever work. Wouldn't it be better to use a zener or a low impedance voltage divider to create a virtual ground for the 1K? The circuit I am referring to now is attached.
 

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Well, Vss is 13.8 VDC. I get a little over 12VDC on the output pin. on the + and - inputs I get a little over 11 VDC. I made sure that the voltage divider for the + input was connected to ground and that all of the connections were made.
In my opinion, there's no plausible explanation for having 11V at this nodes, except for a wrong circuit wiring.
You do realize you chose the "unstable" circuit. Also, 100 pf is way too small.
It's not unstable in terms of bad phase margin (oscillation risk). Also the 100 pF should cause no problem besides reducing the gain at lower frequencies.
Yes I am going to swap in an LM7171.
The expected voltages are mid supply for both inputs and the output, the same as with LM318.
 

I am not so sure. Since the bandwidth of the amp is very low, the additional phase shift from the pole caused by the 100 pF x 1K may push it into instability.
 

By this you mean that I should use a higher one like the .1uF?

---------- Post added at 14:04 ---------- Previous post was at 13:59 ----------

Wouldn't the 100pF charge up to the supply through the feedback resisters and since the 100k's on the + input are so much higher in value and the + and - inputs tend to stay close togeher, wouldn't they be pulled up too? If I lower the 100k's I will loose the advantage of a higher input impedance than an inverting amp, which is why I went with the non-inverting in the first place. That's when all this confusion started. I admit I have a lot to learn about op amps.

Thank you. Can you clarify this for me, please?

---------- Post added at 14:05 ---------- Previous post was at 14:04 ----------

What about using the low impedance voltage divider or zener to create a virtual ground?

---------- Post added at 14:20 ---------- Previous post was at 14:05 ----------

I don't understand the point of that 100pF cap inthe first place. How does that solve the single supply biasing problem? It leaves the 1k feedback resister open and hanging for biasing purposes which makes no sense, and for small signal purposes it shorts it to ground which is the negative supply of the op amp, and that makes no sense to me either. Can sombody please explin this to me? Maybe I should use the other method.
 

I am not so sure. Since the bandwidth of the amp is very low.
The bandwidth of LM318 isn't low. It's about 10 MHz at low supply voltage.

Biasing of the discussed circuit (from post #4) is basically good in my view. The scheme can be often found in single supply operation.

The voltage divider sets the +ve input to Vb/2, 100k is sufficient low resistance for the about 150 nA input current of LM318. The DC feedback factor is unity, in other words the circuit works as a voltage follower for DC, so within it's voltage limits, the OP will "copy" Vb/2 to the output, achieving maximum AC voltage swing.

For AC, the circuit uses the negative supply as it's ground node. Source and load should be also referenced to this node. Except for very low frequencies, where the capacitor values get unhandy, individual decoupling of each amplifier isn't bad. A common "virtual ground" brings up a risk of unwanted coupling between amplifier stages.

If you use e.g. 100 nF instead of 100 pF, you get a corner frequency of about 1.6 kHz for the feedback network, which is O.K. in many cases, for an audio amplifier, you would put in 10 uF.
 
Biasing circuit is neccesary because input stage is differential pair ( probably bipolar) and that's why an amount of DC voltage is used to drive the bases of the input diff. stage.Otherwise it will not work..
But why it does not use its own bias network ?? There should be a reason..
I guess a pre-biasing voltage has been applied to speed up the transient response..
 

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