Re: Op amp help please
- From: John Popelish <jpopelish@xxxxxxxx>
- Date: Mon, 06 Jun 2005 02:17:55 -0400
seware wrote:
I'm fairly new to electronics and I am having a bit of a problem understanding which path to take. I am needing to construct a receiver stage that accepts a differential input signal. So I have been reading about and planning to use an op-amp wired as a difference amplifier.
I'll put off giving you advice about the application till we clear up some of the opamp generalities.
One of my questions is in powering the op-amp... and also understanding enough about different op-amps to make a good selection. I have +12VDC power and am uncertain whether I should use a voltage inverter IC, construct a stage to produce -6 and +6 or what. Then I started reading about single-supply opamps, but I haven't found anything that fully explains them (at least enough to make a choice). I know what "rails" are but what is the difference in "rail-to-rail", "not rail-to-rail" and "outside the rails" which are some terms I found on various chip makers sites.
All these terms relate the voltage range of input and output signals to the supply voltages (rails). All opamps have specific limits on what the input voltages (there are two input terminals) can be and still have the input pair function as a differential amplifier. This is called the common mode voltage (the voltage the two inputs have in common while the differential amplifier amplifies their difference) range. There is also a different input voltage range (related to the supply rails), called absolute maximum voltage limits, that prevents damage to the device. The opamp does not actually care if its rails are on one side of signal common or straddle it, as long as the applied input voltages stay within the absolute limits. It will function as intended as long as the input voltages stay within the common mode voltage range, with respect to its supply rail voltages.
Ordinary opamps have a working common mode voltage that reaches to within a few volts of either supply rail voltage (regardless of what those voltages are with respect to signal common). Single supply types typically work from at, or slightly below, the negative rail voltage to within a few volts of the positive supply rail. Rail-to-rail types have a common mode input range that equals or slightly exceeds the voltage range between the rails.
The output of an opamp is also limited in its voltage swing by the voltages applied to the supply pins. Ordinary (not rail to rail types) typically can swing their outputs within a few volts of either supply rail. Those designated as single supply types can swing their output voltage to within millivolts of the negative rail under some load conditions, but still swing to within only a few volts of the positive rail. Rail-to-rail types can swing their output voltage to very nearly either supply rail (under some load conditions).
So you need to explore your range of input signal voltages and what you can do to bias them in different ways, and your requirement for output voltage swing, before you are ready to try to fit those requirements to a combination of opamp types and available (or practical) supply voltages.
The last part of my confusion come from that fact that I intentionally want to clip any negative portions of the output of the opamp.
This is a separate problem until you decide how this clipping is to be done. The two general ways involve saturating the output (which may have recovery problems and input over voltage problems, when the negative feedback opens up during saturation) and using some nonlinear feedback that abruptly changes the gain as the voltage passes through some particular voltage, but the opamp continues to function as a closed loop process the whole time.
One place I read said that I could use ground as the negative supply to the opamp to accomplish this, but another book I have said that if the input signals go outside the limits of the opamp power supply, the opamp can lock-up.
A definite possibility for some opamp designs.
Which is it and where can I go for a clear understanding? Thanks all and sorry for rambling.
Start with the signals and work out to what you need to do with them, to set some boundaries on the problem. Try to keep as much flexibility in this part of the process as you can imagine, to not rule out some good solutions before they are explored. Then you are in a position to begin considering which of the three general kinds of opamps and their required supplys fit each way the signals could be processed.
.
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