Multi Stage Op Amp Design Questions - Page 1
Forum section: electronics / projects

author: rcbuck
author_rank: Frequent Contributor
timestamp: September 13, 2020, 12:18:49 am
content: I need from anyone who has multi stage op amp design experience. I am trying to design an op amp circuit with high gain. The attached PDF document shows the circuit I am using. All of the parts are SMD parts mounted on a 1-1/2 x 2 inch circuit board. The bottom of the board is a ground plane with only 5 traces on the bottom. None of the traces on the bottom are longer than 1/2 inch.I am using my AWG to inject a 60 kHz, 10 mV signal into the input at PL1. Everything if good up to C11. At the junction of C11 and R9 I have a nice clean sine wave of 2 volts, gain of 200. I am trying to get a gain of 800-1000 if possible.If I move the scope lead to the other side of R9 everything goes to hell. Screen shots are attached that show what is happening.I have tried changing the value of R10 to 47K to make U2a have a gain of 1 but that doesn't change anything. I tried placing a 27 pF cap in parallel with R10 which should form a low pass filter with a cutoff of ~60 kHz but still no change. U1b is a bandpass filter centered at 60 kHz.I tried searching online for tips but didn't really find anything. Suggestions appreciated.

author: bob91343
author_rank: Super Contributor
timestamp: September 13, 2020, 12:35:23 am
content: Obviously you have oscillation. I suggest a careful analysis of the signal paths and returns and bypass capacitors.In general, you need to control all signal paths so that they are where they belong. This is much more than a schematic diagram.Trace the path of each input and output signal from each amplifier. It's obvious that some of the output is being picked up by the input, perhaps due to capacitor current.It would be nice if a bypass capacitor really bypassed. But it doesn't. It just offers a low impedance to high frequencies, and when you construct a circuit you need to use the capacitors to direct both the signal and its return.If a capacitor has an impedance (we must include ESR here) and is placed across two points which should be at the same potential, those points can only be at that potential if a current flows between them. And of course the impedance of the capacitor develops voltage across its impedance and thus does not completely bypass the signal.Your problem is likely in the layout, not choice of components or circuitry. The circuitry has parasitic parameters that have allowed an oscillation due to so much gain.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 13, 2020, 01:03:56 am
content: Hi,There is an 'issue' in the design of the filter U1B. The non-inverting input is connected to +6V. In this circuit +6V is essentially the small signal ground.You can think of the rails being +/-6V and the 6V is ground.However, U1B has R7 connected to ground. This will amplify any difference between ground and the 6V rail.You probably would benefit from connecting the bottom of R7 to +6V.You have the same issue with R11. R11 should be connected to +6V ( the small signal ground).Regards,Jay_Diddy_B

author: rcbuck
author_rank: Frequent Contributor
timestamp: September 13, 2020, 02:02:22 am
content: Bob,Yes, I have some type of oscillation. The scope shows it as being 254 kHz. All signal returns and bypass paths are to the solid ground plane on the bottom of the board. I guess I could try moving the +12 volt supply to the other end of the board where U2 is and see what happens.Photo of board is attached. The distance between U1 and U2 is only 0.7 inch. C11 and R9 take up half of that distance. I have R10 removed for this photo. I don't see how I can make the connection from the output of U1B to the input of U2A any shorter. The only trace on the board of any appreciable length is the +12 volt line. I have 100 nF bypass caps on the supply pin of both U1 and U2. Each of the +6 volt connections to the non-inverting input pins has a 100 nF bypass cap. Do you think it would help to cut the +12 volt trace to U2, insert a 100 ohm resistor, and bypass U2's pin 8 with a 100 uF cap?Jay_Diddy_B, the output of U1B (junction C11 and R9) has a perfect sine wave. I took the filter design straight out of a TI op amp design app note. The only thing I did was change the resistor and capacitor values to match 60 kHz. For what it's worth, the circuit simulates perfectly in LTspice. Clean sine wave out and nice peak at 60 kHz. But obviously real world results are different as it breaks after C11.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 13, 2020, 02:24:29 am
content: Hi,Try the attached LTspice model and you will see what I mean.You will find the circuit is 40dB (100x) sensitive to the input on the bottom of 610resistor or the non-inverting input.The center frequency is 50 kHz with the component values you gave.I have attached the LTspice file.Regards,Jay_Diddy_BFilter test.asc(4.35 kB - downloaded 63 times.)

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 13, 2020, 02:44:11 am
content: Hi,Have you simulated the JFET stage?What is the value of the inductor? (part No. ?)It doesn't look right to me.Jay_Diddy_B

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2020, 03:07:57 am
content: If you could take a nicer screenshot/print that would be great. The DPI in that PDF is... I don't even know how something would make a bitmap that bad?Curious why op-amps, just a few JFETs or BJTs are needed for the same overall gain. And reasonable values of L and C can be used if it's narrowband.What is this for, WWVB..?Tim

author: rcbuck
author_rank: Frequent Contributor
timestamp: September 13, 2020, 04:25:14 am
content: Jay_Diddy_B, I haven't simulated it yet. I will try it tomorrow. However, every single supply filter I find online has the resistor going to ground. Google the phrase I mention below. All of John's filters have the resistor going to ground. The only thing he does different is he has 200k resistors between the 6 volt supply and the non-inverting input of the op amps. Maybe that is key to stopping the oscillation of U2??I'm glad you mentioned the inductor. That is a mistake that I didn't catch when I posted the PDF. The inductor has been replaced by a 4.7k ohm resistor. I couldn't get the FET to work with a 2 mH inductor so I put the resistor in its place. I forgot to update the schematic.T3sI4co1I, Yes, the amp is for WWVB. Google this term: "KD2BD A Frequency Standard for Today's WWVB". You will find a link to the PDF article. I am not building my unit for frequency measuring purposes, only to decode the time data.I am taking ideas from John's article. But I am using a ferrite rod antenna, not a remote loop with a preamp. If I connect my spectrum analyzer to C11 I can clearly see WWVB when they increase the carrier to full power. But I need more gain. I am not going to use the old AM detection. I am going to try to decode the BPSK data. I am using op amps to try to get as clean a signal as possible. I am going to implement AGC in the design so op amps are probably better for that than discrete devices. But I am open to suggestions.I am doing this as a learning exercise. At one time there was an ES-100 module you could buy that did the decoding. A new one is in development. But that is just like putting Lego blocks together. Someone else has done the work for you.I am attaching a new PDF which hopefully will be more readable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2020, 07:59:59 am
content: Ahh, would it not be more straightforward to use a traditional radio receiver design then? For example, cascade a JFET and a few BJT stages, totaling some ridiculous gain at full throttle; vary bias to the first or few stages (or all) to control gain, since gain is proportional to transconductance, which varies with bias current (exponentially to quadratically for the JFET, exponentially for the BJT). It'll definitely consume less supply current than the 5532s!Among op-amps, this can be done directly with operational transconductance amplifiers (most likely LM13700), which, *shrug*, I guess it might save some components versus a discrete solution, mainly for all the bias and decoupling components needed. Though you still need at least a few of them for bandpass filtering.(Note that, to vary the JFET gain, you should probably bias the source up a few volts, using a resistor divider; then gate voltage can be controlled with positive voltage, no negative supply needed. Alternately, use a grounded gate configuration -- assuming impedances are suitable.)Which, speaking of: what is your source? If it's an inductive pickup, the impedance is probably quite low, and a BJT or grounded gate preamp may give a better noise factor.Tim

author: rcbuck
author_rank: Frequent Contributor
timestamp: September 13, 2020, 04:11:03 pm
content: T3sl4co1l, source is the ferrite rod antenna. I have my AWG output connected to a 4 inch, 3 turn loop connected in series with a 50 ohm resistor. I move the loop close enough to the ferrite rod antenna so that I have a 10 mV signal into the gate of the FET. This is much stronger than the 100-200 uV signal that will be coming from WWVB. From what I read the ferrite rod antenna should have an impedance of over 20k ohms. I have caps across it to make it resonate at 60 kHz.This is just to test the front end to see if it works. If I can get the op amp chain to output a 8 to 10 volt signal out of the last stage, I should then be able to see a much stronger WWVB signal. After that I will start working on the AGC and other parts of the receiver.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 13, 2020, 04:45:45 pm
content: Hi,In my opinion the JFET stage should look like this:Note: the damping is provided by the next stage.With a 4.7K resistor instead of the tuned circuit:about 10dB of attenuation.Edit: I have attached the LTspice modelRegards,Jay_Diddy_Bjfet stage.asc(2 kB - downloaded 43 times.)

author: bob91343
author_rank: Super Contributor
timestamp: September 13, 2020, 06:08:08 pm
content: The ground plane might be the problem. No current should flow in a ground plane. All return paths should have integrity even without a ground plane, which should only be used as shielding. To this end, its best to connect a groud plane to your circuit at one point only.This is not simple. Even a solid ground, a lump of solder, has impedance and will cause problems in sensitive applications.

author: rcbuck
author_rank: Frequent Contributor
timestamp: September 13, 2020, 06:21:17 pm
content: Jay_Diddy_B, could you attach your first jfet stage.asc file to a post? I ran your previous Filter test.asc simulation and I see the 40 dB sensitivity difference.I will try resonating the 2 mH inductor and look at the output of the JFET.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 13, 2020, 06:56:33 pm
content: Quote from: rcbuck on September 13, 2020, 06:21:17 pmI have attached the model to my earlier post.Jay_Diddy_B

author: rcbuck
author_rank: Frequent Contributor
timestamp: September 13, 2020, 07:25:45 pm
content: Thank you.

author: rcbuck
author_rank: Frequent Contributor
timestamp: September 14, 2020, 11:24:39 pm
content: I got the op amp circuit working properly with a few changes. The cause of the oscillation at U2 was due to feeding the output of U2a into pin 6 of U2b. Apparently the unity gain amp doesn't like that for some reason.I added a 220k resistor in series with the 6 volt supply to feed pin 5 of U2a. Moving the output of U2a to pin 5 results in a nice clean sine wave out of U2b. I also added a 220k resistor in series with the 6 volt supply feeding pin 3. That is probably not necessary but I did it anyway. When I make the next boards I will put the same 220k in series with pins 3 and 5 of U1. It can't hurt anything.I also changed the gain resistors for U2a. With the original values I was getting close to 6 volts out of U2b. I wanted to keep the voltage to around 4 volts output. The new values accomplish that.I changed the values of C8(originally C7) and C9(originally C8) and the value of R7. This moved the band pass filter much closer to 60 kHz. The peak in now only a couple of hundred Hz above 60 kHz.