Re: current-mode opamps

John Larkin wrote:
I'm still trying to figure out current-mode opamps.

In a conventional opamp, the emitters of an input diff pair are fed by
a current source, and the diff input voltage steers that current. In a
current-mode opamp, the complementary input transistors have a small
quiescent bias, but the non-inverting (bases) input signal invokes
large collector currents, which are amplified all the way to the
output. So in a sense we are getting information from the signal
applied to a regular opamp, but we're extracting power from the input
to a current-mode opamp. Which is why slew rates can be so high: more
error loads the input signal harder, and drives the outputs harder,
without running out of current.

If you look at this,
http://focus.ti.com/lit/ds/symlink/ths3201.pdf

you'll see that the ni input impedance is spec'd as 780K + 1 pF.
They> also provide an S11 plot, which, after it's untangled, models
as a high resistance shunted by about 2 pF or so, presumably
because of extra C on the horribly-laid-out eval board.

OK, I have an open-drain (a phemt) with a 50 ohm pullup to, say,
+2 volts. Unloaded, risetime is 100 ps or so. If it's then coupled
to the THS3201 as a gain=+2 amp, the output falling edge is screaming
fast, but rise is slow. Looking at the fet drain, it looks as if
the pullup resistor is loaded by about 10 pF. Pig city.

So at large-signal swings, the input transistors are running out
of beta as frequency goes up. That makes the input appear to be
capacitive. That's not suggested by the input specs nor by the
small-signal S parameters.

Tricky.

You've got Vin = 2V and G = +2. What are your
two CFB "-input" resistor values?
.

Relevant Pages

  • Re: current-mode opamps
    ... In a conventional opamp, the emitters of an input diff pair are fed by ... current-mode opamp, the complementary input transistors have a small ... OK, I have an open-drain with a 50 ohm pullup to, say, +2 ...
    (sci.electronics.design)
  • Re: current-mode opamps
    ... In a conventional opamp, the emitters of an input diff pair are fed by ... current-mode opamp, the complementary input transistors have a small ... Looking at the fet drain, it looks as if the pullup ...
    (sci.electronics.design)
  • Re: current-mode opamps
    ... In a conventional opamp, the emitters of an input diff pair are fed by ... the pullup resistor is loaded by about 10 pF. ... I had a case a while ago where they could not figure out why a TPS regulator went "exotherm" despite everything around it being well within specs. ...
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  • Re: current-mode opamps
    ... In a conventional opamp, the emitters of an input diff pair are fed by ... OK, I have an open-drain with a 50 ohm pullup to, say, ... the pullup resistor is loaded by about 10 pF. ...
    (sci.electronics.design)
  • Re: current-mode opamps
    ... In a conventional opamp, the emitters of an input diff pair are fed by ... OK, I have an open-drain with a 50 ohm pullup to, say, ... the pullup resistor is loaded by about 10 pF. ...
    (sci.electronics.design)
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