In Part 1 of this post, we looked at the theory involved in making a composite amplifier, but we had yet to replace the ideal amplifier with a real component. Here we will discuss the physical implementation of the second stage.
A current feedback amplifier (CFA) would be ideal for this role. For a quick refresher on CFA’s and how to use them check out my previous post titled “Current feedback amplifier...how do I make it work for me?”
For this high DC precision and high speed composite amplifier, we will select the OPA684 because it has a low quiescent current of 1.7mA, but can achieve excellent bandwidth at high gain. This allows us to optimize the gain bandwidth product (GBWP) of the composite amplifier while maintaining reasonable power consumption for the solution. The OPA684 GBWP is shown below for reference.
Figure 1: OPA684 gain bandwidth product
The circuit with the OPA684 in it is shown below in figure 2.
Figure 2: Composite amplifier with OPA188 as first stage and OPA684 as second stage.
The operating voltage of this solution is limited by the minimum and maximum operating voltage of either amplifier. In this case, the minimum operating voltage is limited by the OPA188 at ±2-V and the maximum is limited by the OPA684 at ±6-V for bipolar supplies.
Figure 3 below shows the small signal AC response for the composite amplifier described here for gains of 10-V/V to 100-V/V. Note that in order to maintain constant bandwidth as the composite amplifier gain is increased, the gain of the OPA684 second stage is increased as well. Since the OPA684 is a CFA with almost constant bandwidth with the gain, the second stage is fast enough to not add any dominant pole in the open-loop gain that would have resulted in decreased stability.
Figure 3: Composite amplifier small signal frequency response
Table 1 below provides the gain and feedback resistors used for the above configurations.
Table 1: Composite amplifier circuits components value
Looking at slew rate, the limiting device will be the OPA188. Even followed by an amplifier in a 100-V/V gain configuration, the maximum slew rate seen at the output of the composite amplifier will be 80-V/us, which is no problem for the OPA684 to achieve with its >750-V/us specification.
This composite amplifier does not achieve excellent bandwidth, only 5.7MHz, but does have the advantage of 580MHz gain bandwidth product [GBWP] in its highest gain configuration and 50MHz GBWP in the lowest gain configuration demonstrated here.