Designing a 6db/octave amplifier

Thread Starter

pyroartist

Joined Oct 9, 2015
88
6db octave amplifier.jpg I found a basic example of this circuit with no values included. I took a first guess at component values and built it on a breadboard. I used the OP221 simply because I have a lot of them. It seems every time I build an op-amp circuit it never works! This one does not work either. It is supposed to be a non-inverting amplifier so it should have a positive output for a positive input. I am looking at the signals with my oscilloscope. The input is correct, the power is present but there is no output. Any ideas?
 

AnalogKid

Joined Aug 1, 2013
8,687
If C1 is 220 uF, then the input filter corner frequency is 0.07 Hz. That's low. It is way below your input signal range, so you will not see any 6 dB/oct effect even if the rest of the circuit works. What is it you are trying to achieve?

Not only does the + input need a DC path to GND (or some other voltage), it needs to be biased into the middle of the opamp's linear range. Same for the - input. Your circuit has the input signal centered around GND, which is the opamp's negative rail. At best the output will be a half-wave rectified version of part of the input. Note that - per the datasheet - the output voltage range is *not* rail-to-rail.

ak
 

Thread Starter

pyroartist

Joined Oct 9, 2015
88
hi pyro,
You must have a DC path to 0v on the non invert pin, try a 47K.
What are C1 and C2 for on that circuit.
E
Eric and everyone. Thanks for pointing this fact out. No wonder ALL of my efforts over the years at op amp experiments NEVER WORKED!
I am quite angry now, at my college text books and teachers for giving me bad information. All my notes do not show correct circuits for non-inverting amps. I will play with some values on the + pin to get it right.
As far as what the capcitors are for I assume it is to get the ~6db per octave gain. If anyone has a way to calculate the resistors and caps for getting this gain curve I would like to see it. Google was of no assistance in finding that. And I don't trust any textbooks now.
 

Thread Starter

pyroartist

Joined Oct 9, 2015
88
I thought that was a confusing value. I've never heard of a 227 nF capacitor. OTOH, 227 is a valid value code: 22 x 10^7 pF = 220 uF.

ak
These capacitors are small (about 3/8 inch square with rounded corners), brown and are labeled "S: 227". I don't think they could possibly be 220 uF. My hand-held meter says they are 0.225 microfarads.
 

Thread Starter

pyroartist

Joined Oct 9, 2015
88
Bordodynov - Thanks! That looks like the gain is increasing over frequency like I want it to.
What is the red dotted line in the top graph? Does that go with the degrees on the right Y axis label?
And what does the red line in the bottom graph represent?
Initially I was trying to make it a non-inverting amp but now I see that because the signal is AC that really does not matter.
Out of curiosity, what components would change to make this a 12db/octave amp?
 

Thread Starter

pyroartist

Joined Oct 9, 2015
88
As eric noted, all op amps need a DC path at their inputs for proper operation.
Does that apply to Both inputs or just the non-inverting input as Eric said above? If it is both then how is Bordodynov's circuit correct as it doesn't seem to have a DC path on the inverting input?
 

OBW0549

Joined Mar 2, 2015
3,566
Does that apply to Both inputs or just the non-inverting input as Eric said above?
Both inputs. ALWAYS.

If it is both then how is Bordodynov's circuit correct as it doesn't seem to have a DC path on the inverting input?
There is indeed a DC path-- through R1 to the op amp's output. Note that it doesn't matter where the path for input bias current goes to, so long as it conducts direct current. Bottom line: never leave an op amp input disconnected, or connected to nothing but a capacitor.
 

Bordodynov

Joined May 20, 2015
2,697
Bordodynov - Thanks! That looks like the gain is increasing over frequency like I want it to.
What is the red dotted line in the top graph? Does that go with the degrees on the right Y axis label?
And what does the red line in the bottom graph represent?
Initially I was trying to make it a non-inverting amp but now I see that because the signal is AC that really does not matter.
Out of curiosity, what components would change to make this a 12db/octave amp?
1. Dotted line is the phase of the output signal. The phase Y axis is on the right.
2. In the second picture, the result of parametric analysis. The parameter is the signal frequency (.step param Fre list 100Hz 2kHz). The red signal is 100Hz.
3. The transfer coefficient is proportional to the value of the resistor R1.
4. To get 12dB/Octave, you need to use two stages. But you have to reduce the gain of each stage or use a small signal (the gain of 2 kHz will be large if you do not take measures to reduce the gain).
 
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