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Electronics Forum / Basics / February 2006Help needed with use of NE5534 op amp in -ve tracking supply!
Hi! This is a follow on from my previous post regarding +- 15 V regulated supply. As per suggestion from this good group - I built a positive supply using a LM 723. Works fine. Now, I have added an inverter follower using an NE5534 op amp. Why NE5534? That is the only opamp I had in my box and besides I want to use a low noise op amp. The op amps power V+ and V- are the 0 and -20.5 output from the filter. The Vin+ is connected to 0 and the Vin - has an input resistor of 33K and a feedback resistor of 33K (Unity gain) with a 2K trimpot in between for adjustment. I have a PNP pass transistor. RESULT: NO OUTPUT - or - rather -0.07 V output If I connect a 3.2K to Pin 1 (Balance) and a 3.2K to Pin 8 (Bal/COMP) - hey presto - everything springs to life! What is going on? Is the Op Amp powered incorrectly? Should I take Pin 7 (VCC+) to the +20.5? If I have to do that - I have to use zeners or something so that the IC does not see more than +- 15V. To +15 Output from the +ve side of the regulator /|\ | NB: There is a 22pF Comp cap between (5) and (8) _____|____________________________________0 | | 33K 2K | 33K | |__/\/\/\__/\/\__|_/\/\/\_ | | | | | | (7)| | | | |\| | | IN- |_ | \ (6) /E | (2) | \____|/ | | / |\ |_________________ | / \C IN+ |/|(4) | (3) | | ---------------------------------------- - 20.5 Please can anyone help me understand this? To me it seems that may be the input transistors are not in their active region because the bias current is too low? Maybe I should lower the 33Ks? Thanks in advance to anyone who even reads this! Any suggestions will be a bonus! Regards Ramendra > Hi! > This is a follow on from my previous post regarding +- 15 V regulated [quoted text clipped - 33 lines] > the input transistors are not in their active region because the bias > current is too low? Maybe I should lower the 33Ks? The only way this could work would be if the input common mode range included the positive rail. The 5534 doesn't. Power the opamp from the +15 volt output, instead of 0V. http://www.onsemi.com/pub/Collateral/NE5534-D.PDF > Hi! > This is a follow on from my previous post regarding +- 15 V regulated [quoted text clipped - 37 lines] > > Any suggestions will be a bonus! For starters, you've connected the 5534 '+ input' to the positive supply rail. That's outside the operational parameters ( see input common mode voltage range on the data sheet ). Therefore it won't work properly. Why aren't you running the 5534 off the +/-15V regulated supply ? What are you actually trying to do ? It normally helps to state this at the beginning. Graham >>Hi! >>This is a follow on from my previous post regarding +- 15 V regulated [quoted text clipped - 48 lines] > > Graham Thanks for your advice guys. Hmmm... ok, it shows I am a novice. Lesson learnt - I should have read the data sheet more carefully. It was all there. Graham, the 5534 is part of the regulated supply. My plan is to use it as an inverting follower of unity gain to track the +ve regulated rail. That is why I did not use the +/- 15V regulated supply. Please see the modified circuit above. Your suggestion should work - I am going to try it. Once again, thanks a lot for your help. Ramendra Texas Instruments makes some extremely low noise op amps that use the so-called chopper technology, the TLC2654C series. These achieve input noise of about 2uV p-p right down to DC. They are available in the old PDIP package which will plug into the Radio Shack protoboards for breadboarding. Keep in mind the TLC2654 is rated for +/- 8V supplies only. Use voltage regulators to keep supplies below +/-8V. TI has a generous, free sample program. Register and they will FedEx you up to 5 samples overnight. The TLC2654 has a bandwidth of about 300KHz so it can be used as a music amplifier for modest gains (5 or less). In particular the low-frequency region (< 1KHz is dead quiet). Charles Gilbert Consultant NonDigital.Netfirms.com > Texas Instruments makes some extremely low noise op amps that use the > so-called chopper technology, the TLC2654C series. [quoted text clipped - 12 lines] > Consultant > NonDigital.Netfirms.com Thanks for your suggestion. I am only a hobbyist. I do build some stuff for people I know and they reward me handsomely. Are these samples meant for anybody or for businesses and industry? Dont get me wrong. I like free stuff too - especially for experimenting. Just want to make sure this is ok. Thanks Ramendra > Hi! > This is a follow on from my previous post regarding +- 15 V regulated > supply. As per suggestion from this good group - I built a positive supply > using a LM 723. Works fine. > Now, I have added an inverter follower using an NE5534 op amp. [...] Others have addressed the question of why it isn't working. I want to ask: do you actually need it in the first place? Tracking supplies are actually a rather specialized thing. There are very few cases where they are needed, and in most cases they are actually more of a problem than a benefit. In many cases, what you actually want is just a dual regulated supply, where the -ve and +ve supplies are separately regulated and do not track each other. This does mean that they won't usually be exactly balanced around zero (e.g. it might be +15.1v and -14.8v or some such) but for most circuitry that is not a problem. The benefit of dual non-tracking supplies is simpler circuitry, better stability, and lower noise: unless you are careful about filtering, a tracking regulator takes whatever noise is on the master side and adds it to the slave side. That noise could be coming from the circuit being powered - for instance, digital switching hash on the +ve supply can get copied over onto the negative supply. Not, in general, what you want. >>Hi! >>This is a follow on from my previous post regarding +- 15 V regulated [quoted text clipped - 21 lines] > for instance, digital switching hash on the +ve supply can get copied over > onto the negative supply. Not, in general, what you want. Thanks for your response. I hadnt looked at any of the points you have mentioned. I am making a tracking supply to test it and to study how it works. That will take me quite some time. After that I wish to use it as a standard supply for other circuits that I build. I want the +ve and -ve rails to be balanced so that I can assume that that they are whilst I work with my other test circuits. The first of these is going to be sine wave generators of different kinds. I am starting up a hobby I used to have a long time ago. Lots of things have changed - I am slowly starting to re familiarise myself. I have built a dual supply using two separate +ve supplies running off two secondaries of a transformer. Now, I am working on this tracking supply. Thanks once again for raising the points you have raised. I will try to incorporate a RC filter where I am sampling the +ve rail to try and exclude noise from being replicated by the negative follower as you have said. I hopr that works. Ramendra |
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