Re: current-mode opamps

On Sun, 24 Feb 2008 18:09:24 -0800 (PST), Winfield Hill
<hill@xxxxxxxxxxx> wrote:

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?


At G=2, 100 ohm load, they suggest that both resistors be 715 ohms for
good large-signal performance. Playing with the resistors changes
transient response but doesn't much affect the apparent 10 pF input
capacitance.

The eval board, and the S-params setup, effectively drive the ni input
from a 25 ohm source. The frequency response curves are unclear on
what the generator source impedance may be. They give inverting and ni
curves small-signal, but the large-signal stuff is all inverting. Are
they trying to hide the actual performance?

TI is beginning to disappoint me.

John


.

Relevant Pages

  • 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)
  • Re: current-mode opamps
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