Here's An Easy Way To Test Wideband Transimpedance Amplifiers

Substituting the values from Figure 3 predicts a measured gain of 10.53 V/V (or 20.45 dB). The network analyzer will actually measure the gain after the 6-dB loss is taken through the series matching resistor at the output. Output matching is optional for this application but very useful for network analyzer measurements. Since the OPA658 will deliver this output signal, and it's optimized to drive 100-Ω loads with no loss in performance, this should not introduce errors into the measurement.

The 6-dB loss through this output matching resistor can be normalized out by performing a "thru" calibration of the analyzer by itself. Do this by connecting the output of V_s directly into the measurement port with a cable and performing a "thru" calibration. This will normalize out this last 6- dB loss and any nonuniformities in the signal source or measurement port over the test frequency.

Figure 4 shows the measured gain and phase for the transimpedance gain stage plus the test interface of Figure 3. These measured results very closely match those predicted by the analysis. The increasing portion of the gain curve comes from the zero in the transconductance of the test interface circuit as shown by Equation 1.

The frequency response for just the transimpedance amplifier will continue flat down to dc at the midband gain of Figure 4 (shown as a dotted line). The measured low-frequency corner occurs exactly at the predicted 1 MHz point. The measured 23.7 dB midband gain is very close to the predicted 23.45 dB. The difference is attributable to component tolerances.

The measured high-frequency corner comes at a slightly higher frequency than predicted--17.2 MHz vs. a predicted 14 MHz. This is likely due to the additional phase shift inside the loop introduced by the OPA658 gain stage. Even though this amplifier has a small signal bandwidth greater than 200 MHz at the gain of +9 used here, its 0.8-ns propagation delay introduces an additional 8° phase shift at the loop-gain crossover frequency of 28 MHz. This slight degradation in phase margin is extending the closed-loop bandwidth.