DC - 5 MHz BW 40 dB Amplifier

M. Gallant Sept 27, 2009

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.]

4 MHz / 26 dB Gain Very Low Offset Circuit

If less gain but higher bandwidth is required, the circuit above can be easily modified to achieve a higher bandwidth of 25 MHz at a reduced gain of 26 dB (x20) by replacing Ri in the circuit above with 270 ohm. The high-speed bipolar opamps such as the LT1222 tend to have fairly high input bias currents (100 nA in this case). With an input effective source resistance of 5kohm (combined R6 and Rs) and a gain of 100/20, this can lead to output offsets of ~ 100/20 mV which will be source resistance dependent and which cannot be nulled. For the lower gain case of 26 dB, a high performance DiFET op-amp such as the OPA637 (GBW 80MHz, SLR 135 V/us) will provide a 4 MHz bandwidth with 3V peak output before slew-limiting occurs. This configuration will have negligible contribution in output offset due to the input bias current of ~ 1pA and the ~ 1 mV output offset voltage (50 uV input Vos) can be nulled with a trimmer to ~ 100uV. Output noise level in this 26 dB circuit will be about 600 uVrms.