# Can I just use one end of a differential op-amp ? (ie use differential op-amp as a single ended op-amp)

I found this awesome op-amp I can use which is super cheap and super fast (NE592D).

Unfortunately, it has differential inputs and outputs. I can probably deal with the differential inputs, but how can I convert the differential output into a single ended output?

Can I just put one end to ground? or to ground via a DC blocking cap? or even just not connect one side?

You can just ignore one of the outputs if you only need a single ended output. Leave it open.

It is rather an antique IC, something like 4O years old. What characteristics of the amplifier are so good for your application?

You can't use it as a general purpose amplifier - it's bias current is high; its offset voltage is not very good. You probably won't be able to use feedback without stability issues.

There are probably better ones available now.

• Fast n cheap. I want to connect it to a coil, which will pick up the emissions of a RFID reader. I can just connect both ends of the coil to the differential input, set up the gain, and use one end as the input to an envelope detector. I hope. – 64bit_twitchyliquid Mar 3 '17 at 3:01
• I agree with @Kevin, you really should look to more modern amplifiers. Looking at Digikey, you can get the Micrel MIC920 for only $0.34 and while it has only 80 MHz G/BW product, it looks to easily fit your needs. Then there is the TI LMV793 for only$0.57 with a G/BW of 88 MHz. – Jack Creasey Mar 3 '17 at 5:32

The NE592 has an equivalent input noise of 10 microVolts RMS in 10MHz bandwidth (per ONNN SEMI datasheet). In one Hertz BW, we divide 10uV/sqrt(10^7) to find 3.16nanoVolt noise density, or Rnoise of 600 ohms.

The MIC920 has 11nanoVolt noise density, or Rnoise of 25,000 Ohms. Thus the MIC920 costs 10dB or more of SignalNoiseRatio, and needs external components to set the gain. But MIC920 has lots more output drive ability.

If you want the NE592, just leave one side open. Be gentle in demanding current to charge a peak-detector thru a diode. The NE592 has internal feedback to prior (internal) stage, implementing a Cherry-Hopper gain-stage for linearity. Capacitive loads upset the delay (the phase) and may produce peaking or oscillation. But such is a risk for any "opamp" driving a peak detector.

You can make an envelope detector out of NPN, base biased barely on with a similar NPN, resistor in emitter to linearize the rectification response, and a R+C collector load to set gain and bandwidth (to track modulation).

• RFID is not a particularly challenging low signal/noise application. I'd assume the noise floor would not be of concern when the input is a coil in close coupling to the RFID card. The NE592 is single sourced to ON now, and they have everything except the SOIC on life support. Not a good long term bet I'd say. – Jack Creasey Mar 3 '17 at 6:34