DC - 5 MHz BW 40 dB Amplifier
M. Gallant Dec 14, 2008
The schematic diagram below demonstrates a 40 dB (gain of 100) DC to 5 MHz bandwidth low-noise op amp circuit which can
be used as a general purpose instrumentation gain block. For example, it can be used as a voltage
preamplifier for medium bandwidth digital oscilloscopes such as the Syscomp DSO-101 or as a DC/AC preamplifier with any
multimeter to extend the sensitivity range.
To achieve this gain at 5 MHz requires an op amp with a GBW of at least 500 MHz.
To achieve say 5 Volts peak output level without distortion at 5 MHz requires a slew rate of at least 150 V/us.
A suitable choice is the
LT1222
op amp, with a GBW of 500 MHz, a slew rate of 200V/us, low voltage noise and stable in a gain of 10 uncompensated.
As usual, good power supply bypass capacitors and short leads are required. Ground return points should be
as close together as possible.
The circuit below shows the essential components.
Note that the LT1222 spec sheet recommends a maximum Rf of 5 kohm for best performance and stability.
The noninverting input was terminated with a 4.7k resistor. The gain (both signal and noise) for this stage is (Rf + Ri)/Ri,
which for the component values shown is 40.1 dB (Av = 101). The 10 kohm voltage offset adjustment potentiomenter
is only needed if accurate DC measurements are required.
The breadboarded circuit below provides a rudimentary test setup for component placement and noise-critical placement.
The mini-breadboard was mounted in a small aluminum box with suitable connectors. With reasonable care in layout,
a circuit implementation with low noise and stable against oscillation can be achieved.
The response curves below show dual-channel traces at 100 kHz, 1 MHz and 3 MHz frequencies indicating a 3db bandwidth
of about 5 MHz. The drop in signal level above 1 MHz is due to the signal test source used.
With a 4.7kohm input termination, and without input source connections, the amplified output noise voltage in a bandwidth
of 5 MHz (or a noise-bandwidth of 5x1.57 = 7.85 MHz) is about 4 mV rms. This is approximately
the noise level expected for this circuit and is dominated by both the op amp bias current noise and the thermal noise of 4.7kohm.
[With an input termination source resistance of 100 ohm, the output noise voltage would be about 1 mV rms, dominated by the LT1222 op amp
input noise voltage.]
The circuit above can be easily modified to achieve a higher bandwidth of 50 MHz at a reduced gain of 20 dB (x10) by replacing Rf with 470 ohm. In this case,
the peak output voltage achieveable before slew-limiting is reached is reduced to about 0.5Vp.