Need opamp / comparator advice

[CaptDon]

I have an old Nems Clarke GPR-20 general purpose receiver. I believe it to be all PNP transistors as it uses -12 volts referenced to chassis ground. I would like to add a C.O.R. (Carrier Operated Relay). The signal strength meter is chassis grounded on its positive connection and the negative connection is around -5mv at no signal and goes to -100mv at full scale 100% reading. Signals producing -10mv or more on the meter terminal are useful with an acceptable signal to noise ratio. I am wondering if a rail-to-rail opamp would work. I would set it up open loop as a comparator with the voltage trip point adjustable on the + non-inverting input and the negative going signal strength signal on the - inverting input. The output shall drive a relay rated at 20ma@12vdc. Can a rail-to-rail device work properly with input signals this close to the rail voltage? I would be hooking up the opamp / comparator 'upside down' so to speak with the positive supply pin referenced to chassis and the negative supply pin connected to -12vdc. Any suggestions as to what DIP-8 device I should choose? I can't do surface mount devices at home so I would be looking at a DIP-8 indeed. Thanks!!

[Manul]

Yeah, most op amps are not designed for common mode range up to or above V+. I know a series of op amps from analog, like a LT1637, they call them Over-The-Top (what a gem of marketing). These have very wide common mode range. Otherwise I don't know, you can try a parametric search.

[Kleinstein]

The MCP6006 gives data for operation even some 200 mV above and below the supply range. Most, especially older datasheets don't really specify the behavior slight outside the supply, but there is little reason to asume it should not work.  Chances are the even the LM358 will work a little below its negative supply at the input, though not at the output.

[CaptDon]

I guess my explaination left some doubt as to voltages. The Positive supply pin of the opamp shall be at chassis ground, the negative supply pin of the opamp shall be connected to -12vdc. The + non-inverting input will be adjustable from -10mv below chassis ground (A.K.A. the positive supply voltage of the opamp) to around -50mv below chassis ground and the - inverting input will be directly driven from the signal strength meter which varies from -5mv to -100mv below chassis ground. When the negative voltage on the - input pin goes more negative than the reference voltage on the + input pin the output will swing to the opamps negative supply rail which in this case is -12vdc. I can add a driver transistor to the opamp output to drive the relay if the opamp can't produce 20 milliamps. In no case will any input voltage be beyond the supply voltage. The inputs will be between 10mv and 100mv below the opamps positive rail voltage which in this radio also happens to be chassis ground. I am wondering can any rail to rail opamp / comparator work effectively / reliably with inputs that are only 10 to 100mv below the positive supply? Thanks!!

[Manul]

If input common mode voltage range in datasheet includes V+ or above (over full temperature range) then it should work.

[Kleinstein]

For the input side rail to rail includes the rails and usually also a little (like 100-300 mV) beyound the rails. It is only at the output where rail to rail may still leave out some 10 or 50 mV (e.g. CE saturation voltage of a BJT) and at least some resistance. So most RR op-amps should work, iif they allow for 12 V supply.

[Ian.M]

You'll want a stabilised voltage to divide down for the reference level, so maybe use a 79L05 regulator to power the OPAMP, which opens up the choice to all the 5V RRIO OPAMPs.  You'd then need a PNP transistor to drive the relay, emitter to Gnd, base with a resistor to the OPAMP output.

[MarkT]

Can a rail-to-rail device work properly with input signals this close to the rail voltage?

If you want a comparator (positive feedback), use a comparator, if you want negative feedback (amplification etc), use an opamp.

You can sometimes use an opamp as a comparator, but its not ideal and many types of opamp cannot handle this and will be fried, you'll need to read the datasheet in detail to figure out the ramifications of such alternate use...

LM339's and similar are very cheap and purpose-made as comparators...

BTW Comparators cannot work as opamps ever, note, they are completely unstable under negative feedback.

[Ian.M]

Unfortunately the LM339 common mode range does not include its positive supply.    In this application, comparing a signal that is between 5mV and 100mV below the +ve rail against a threshold voltage in the same range, with a 12V supply,  there isn't a lot of choice for comparators.   Again, adding a -5V regulator opens up the choices considerably.

[Vovk_Z]

Again, adding a -5V regulator opens up the choices considerably.

He has now -12 and 0 VDC rails. So, as I could understand, he needs rather +5 or +12 VDC rail.

[Ian.M]

A -5V regulator running from the -12V rail allows the use of modern 5.5V max supply comparators and OPAMPs, more of which have  rail to rail inputs.   The down-side is the need for a low Vgs threshold P-MOSFET or PNP transistor to drive the relay, but it probably needed a driver transistor anyway unless the relay has very low coil current.

He has now -12 and 0 VDC rails. So, as I could understand, he needs rather +5 or +12 VDC rail.

A +5V rail would be nice to have but if all you've got is a negative supply, it would certainly be a bad idea to add a DC-DC switched mode converter to get one, in a highly noise sensitive comms receiver!

[David Hess]

Most of the old JFET operational amplifiers have an input range which includes their positive supply, like the TL031, TL051, TL061, TL071, LF351, and LF411.  The old bipolar LM301A also has an input range which includes the positive supply.

Modern rail-to-rail input parts will of course also work.  Suitable low cost modern parts include the OPA990, OP991, OPA196, and OPA197.