Chris Jones
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 Posted:
Sun Oct 02, 2005 12:35 am Post subject:
Re: Buffer Amp IC suggestions plse |
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Winfield Hill wrote:
| Quote: | Chris Jones wrote...
Winfield Hill wrote:
Jeff wrote...
Needs to drive into 50 ohms. input level approx 150mV at 5Mhz
Winfield Hill wrote ...
Jeff wrote...
Ive been trying to locate a suitable unity gain buffer amplifier
IC that can drive a 50 load. Application is to buffer a VCO
output at 5Mhz. Needs to be 8pin DIP
LTC's LT1206 comes in TO220, miniDIP and soic packages,
and operate on a single-ended supply.
Oops, +/-5V supply. What's your signal voltage? And did
you want your output to have a 50-ohm source impedance?
Unless you source with 50 ohms (2x signal plus 50-ohm resistor),
and terminate with 50-ohms, the issue becomes how much cable
capacitance? Either the load looks resistive (100 ohms in the
former case) or capacitive.
I would say it depends on the cable you choose. If there is a 50 Ohm
load and you connect it to your buffer amplifier with 50 Ohm coaxial
cable, then the buffer amplifier will see a 50 Ohm resistive load,
not capacitive at all.
It is probably still wise to use the gain-of-two amplifier and 50 Ohm
source resistor in case the load happens to be a poor match. If you
really don't need this feature then I would be tempted to use an
emitter follower with just one transistor. (Do they make transistors
in DIP-8 packages?)
I agree, if the load has no capacitance, source terminating with
50-ohms eliminates the cable-capacitance issue, and the driver only
sees a 100-ohm load at high frequencies, above those where the open-
end reflection has time to return. Although the input to the cable
is attenuated by 2x (hence the 2x gain you suggested), the end of
the cable gets the 2x back due to the open circuit, so a gain-of-two
amplifier shouldn't be needed. Just the 50-ohm sourcing resistor,
after the emitter follower.
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What I meant to say is that there is no need for a resistor at the amplifier
end of the cable, provided we know for certain that the load is resistive
and of the right impedance to terminate the cable, i.e. accurately 50 Ohms.
If this is the case, then there is no need for an amplifier with voltage
gain, which might make thing easier. If the load is not close to 50 Ohms
and if we care about the frequency response, then terminating both ends of
the cable makes a lot of sense.
There is a more important question which the OP could answer which is: what
is this whole thing for? Is the VCO incapable of providing enough current
to drive the cable, or is the amplifier there to provide reverse isolation
so that the load cannot affect the frequency, or is there another reason
why we need this buffer amplifier? Anyway at 5MHz I would probably try to
do it with one or two transistors rather than buying a chip, since the
continued availability of transistors is more likely.
Chris
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