Hello, I am using the AD831 in my design, WITHOUT using the internal amplifier (using the differential current output) and I have a couple questions.

1. The internal gain indicated in the datasheet (pi/2) is provided by the gilbert cell or the output amplifier?

2. What is the maximum load resistor I can put in the differential output for correct biasing of the AD831 internal transistors? (I am using the lowest bias current) For example other gilbert cell mixers establish a minimum Vce of 2V for a correct operation.

Thanks.

Julio Martin-Hidalgo

• Hi Julio,

1)  The pi/2 is the small signal gain of the whole signal chain, including the I-V output amplifier.  The gain should be a similar exercise to a common emitter  with resistive degeneration: Gain = Rc/(Re +1/gm)

2)  I wouldn't go any larger than 200 Ohm load resistors.  You may have to play around with this though and confirm that the transistors are not becoming saturated.  I think a good proxy for this would be to test for IP3 and see if it is still in spec.

Hope this helps,

Joel

• Yep, it sure does help.

About 1), I thought that, but something in the datasheet confused me: when they attach a transformer at the differential output and they say it has unity-gain ¿?

About 2), unless someone comes and tells me they have tried the same, and gives me an operational limit..., I'll do exactly as you say, try a couple values.

Thanks!

• Hi Julio,

Assuming a 50 Ohm load impedance on the IF connector in Figure 2, then a 1:1 balun would reflect a 50 Ohm differential load impedance across the balun, and then it would be 25 Ohms single-ended.  So Rc = 25 Ohms, Re = 20 Ohms, and 1/gm = 1.44 Ohms with 18 mA of DC current, yields a gain of about 1.16, which is pretty close to unity gain.  Remember these are approximate numbers off of an idealized schematic.

Thanks,

Joel

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Thank you,