[SI-LIST] Re: Op-Amp
- From: "o. laney" <olaney@xxxxxxxx>
- To: otter30@xxxxxxxxxxxxx
- Date: Thu, 4 Mar 2010 09:12:21 -0800
There are many schemes that are the responsibility of EEs such as
ourselves to invent, but one I use is to simply cascade a PNP emitter
follower into an NPN one (or vice versa). That way the two Vbe's offset
each other. It isn't perfect cancellation, but if a few tens of
millivolts of residual offset can't be tolerated then an emitter follower
isn't the solution.
Another issue is that emitter followers have gains approximating but less
than 1. Cascading two will result in a gain somewhere around .93 to .97
for transistors with decent beta. And of course, gain drops with
frequency so it might be necessary to add equalization components to push
out response for wideband use. Note that PNP transistors tend to have
lower Ft than their NPN counterparts so they will tend to dominate the
frequency response if you are pushing right to the edge. PNP
UHF/microwave transistors are rare.
That said, I have obtained decent, fast, (nearly) unity gain video &
small signal buffers for pennies by cascading 2N3906 and 2N3904
transistors. For large voltage swings and low impedances, larger
transistors are indicated and power dissipation can be an issue, but
that's true for catalog buffers as well.
A hybrid solution is to drive the follower with an op-amp, taking
feedback from the emitter resistor. The op-amp control loop cancels the
offset and compensates the gain so that you have the best of both worlds:
op-amp accuracy and a relatively inexpensive discrete transistor to
handle the high power. The op-amp + transistor combination can be lots
cheaper than a high power op-amp. The op-amp can also compensate for the
rather less controlled threshold voltage of a FET, making these an option
as well. Because the transistor emitter can only source (or sink)
current, make sure the emitter pull down resistor carries enough current
for the negative swings, especially if the load is capacitive. There are
schemes to use another transistor to provide low impedance pull down
without high idling current, but at that point you are starting to design
another op-amp.
Orin
On Thu, 4 Mar 2010 08:38:29 -0500 "Jim Hall" <otter30@xxxxxxxxxxxxx>
writes:
>
> Hello Orin,
>
> Could you point me toward a reference on how to bias a follower to
> compensate for the Vbe drop? That sounds like a useful thing to
> know.
>
> Thanks,
> Jim
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of o. laney
> Sent: Thursday, March 04, 2010 1:04 AM
> To: codymiller@xxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Op-Amp
>
> Sometimes there is charm in a simple emitter follower,
> notwithstanding
> that biasing a simple transistor to compensate for the diode drop
> seems
> to be vanishing knowledge. You would want an Ft around 1.5 GHz or a
> bit
> higher and layout to match, but not lots higher unless you are into
> impromptu oscillator design. Stability will depend on proper
> termination. Are you sure you wouldn't prefer a gain of 2 with a
> backmatch resistor in series?
>
> Orin Laney
>
>
> On Wed, 3 Mar 2010 08:51:31 -0700 <codymiller@xxxxxxxxxx> writes:
> > All,
> >
> > I am looking for a unity gain amplifier solution that has a
> > bandwidth of
> > 500 MHz or higher if possible. I need to be able to drive a 50 ohm
>
> > load.
> > The voltage range needed is around +-5V maybe smaller if
> necessary.
> >
> > Any component/circuit suggestions would be appreciated.
> >
> > Thanks,
> > Cody
> >
> >
> >
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