Most op-amps these days have a gain-bandwidth product of multiple
megahertz, and so would be suitable.
If I were make this, I would use such an op-amp. I usually favor an
inverting configuration to keep the op-amp inputs from swinging with the
input voltage. You would want to have a 10 kohm resistor from the amplifier
input connector to the inverting input (to give the correct 10 kohm input
impedance), and 20 kohms (yes!) from the inverting input to the output.
This yields a gain of 2 amplifier. However, from the output of the op-amp
to the output connector of the amplifier, you want 50 ohms to provide the
output matching impedance. This along with the 50 ohm input impedance of
your receiver forms a 2:1 voltage divider, so you are back down to a gain
of 1.
If you run the op-amp off of + and - voltages, you can just ground the
non-inverting input to the mid-point of the voltages (also tied to ground).
If you want to run off of a single supply, you would divide the supply
voltage by about 2 with some resistors and tie the non-inverting input to
the mid-point. Note that this will cause that same mid-point voltage to
appear as on offset voltage on the input and output of the amplifier, so
then you need some blocking capacitors, selected to pass 20 Hz without
excessive attenuation. A third alternative is to float the battery, split
the voltage, and tie the mid-point voltage to ground.
For battery operated stuff, I also like to include an LED as a reminder
that it is turned on. Modern LEDs can be run with as little as 1 mA (small
load on battery) and still appear bright to the eye.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA, USA
Ken Javor
<ken.javor@emccom
pliance.com> To
Sent by: Untitled <[email protected]>
[email protected] cc
Subject
04/13/2009 10:05 Audio pre-amp needed
PM
Looking for an inexpensive small audio unity gain emitter-follower type
buffer amplifier with input impedance equal to or higher than 10 K Ohms,
and output impedance of 50 Ohms. Frequency range of 20 Hz to at least 50
kHz with less than 1 dB variation over that range.
If there is some op-amp that can be easily configured to do this, that
would be fine. An off-the-shelf package would also work.
Single channel. Application is a very old EMI current probe amplifier that
has a gain curve that is the inverse of the current probe’s transfer
impedance. The combination of the two yields a flat transfer impedance of
0 dB Ohms from 30 Hz to 50 kHz. The rub is this was designed for use with
a specific obsolete EMI receiver with selectable input impedance. The
required load on the current probe amplifier is 10 k Ohms, which was one of
the choices with the old EMI receiver. Nowadays everything is 50 Ohms, so
I have to provide the matching. I have been doing that with another
obsolete piece of equipment, a Wavetek 452 active filter, but it is a 19”
rack-mount sized piece of equipment, and I would like to make this thing as
small as possible.
Thanks in advance,
Ken Javor
Phone: (256) 650-5261
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This message is from the IEEE Product Safety Engineering Society emc-pstc
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