Re: current-mode opamps
- From: Jim Thompson <To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx>
- Date: Tue, 26 Feb 2008 09:22:30 -0700
On Sun, 24 Feb 2008 15:33:15 -0800, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> 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.
John
The best design that CAD can provide... "it works in simulation",
"designed" by some engineer who has no soldering iron scars ;-)
...Jim Thompson
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