Build High-GBW Op-Amp From A Dual Video Amplifier
The individual video amplifiers are unity-gain stable without external compensation, and have unity-gains bandwidth of 72MHz. They operate on ±5V supplies and consume about 350mW as a pair. Other typical specs include 2mV input offset, 100pA input bias currents, -72dB isolation between amplifiers, and differential phase and gain errors of 0.2° and 0.5%. The dual-amplifier chip comes in an 8-lead DIP.
Figure 1. This composite amplifier, based on a dual video-amplifier IC, provides a 40dB gain and 10MHz bandwidth while driving a 150Ω load.
The approach taken in Figure 1 minimizes output distortion. The first amplifier, operating with no (DC) load, provides it's maximum voltage gain of about 660V/V. The second amplifier, drivng a 150Ω load, has a voltage gain of about 65. Total open-loopgain for the composite amplifiers is thus 660×65, or about 92dB. In the absence of phase-compensation componenets R1 and C3, however, the circuit will oscillate.
With the 150Ω load, output swing is typically ±3.3V and linearity (between -2V and 2V) is about ±0.5%. R1 and C3 adjust the -3dB frequency to 10MHz, giving a gain-bandwidth product of 1GHz. The amplifier provides a 2V output swing with only 3% peaking near the 10MHz rolloff frequency. For comparison, a BB3554 or AD3554 operating at 40dB with no phase compensation (for maximum bandwidth) has a -3dB bandwidth of only 7MHz.
Table 1. Input and output voltages.
(Offset voltage not adjusted)
The table in Figure 1 gives DC performance for the closed-loop amplifier. Gain resistors R2 and R3 measured 99.78 and 9965Ω, giving a theoretical gain of R2 + R3 / R3 = 1000.870V/V—which is very close to the value of 100.825 measured for range -2V to 2V.