With both current and voltage feedback operational amplifiers (op amps) available to the system designer, how do you select which device to use? This two-part article discusses applications most suited to each type of op amp, and why certain applications are unsuitable for one or the other type of amplifier. Widely-used circuits are shown with example designs. The emerging fully differential amplifier type of amplifier (a special case of a voltage feedback device) is also shown along with several suitable applications. Part 1 reviews the internal differences between the two types of op amps and presents some key applications most suitable to voltage feedback type devices.
Initial amplifier selection criteria
There is a bewildering array of possible op amps for a designer to select from in any given application. However, there are distinct internal differences between the two major types of amplifiers: voltage feedback (VFB) and current feedback (CFB). These differences may lead a designer to choose one over the other in certain applications. A newer type of op amp, the fully differential amplifier (FDA), is a type of voltage-feedback op amp that includes an output common-mode control loop. It has the same application emphasis as a standard voltage feedback amplifier, as well as several specific applications ideally suited to this type of amplifier.
Before giving any attention to the type of amplifier to use, most designers first should consider the output signal requirements (for example, maximum desired output Vpp and output current, as well as load type) and necessary dynamic characteristics such as settling time, full power bandwidth, or a distortion requirement. Using these specifications, the universe of possible amplifiers can be narrowed to devices that can handle the correct range of supply voltages needed to deliver the output Vpp, and then limited further to a minimum supply current versus desired output dynamic characteristic. Normally, a CFB gives the best power efficiency for higher-frequency dynamic range needs, while a VFB generally has better noise and/or DC precision, and is the part of choice for certain types of circuits.
Internal comparison of voltage and current feedback op amps
To understand why some circuits work better with one or the other type of amplifier, you need to first understand the internal topology of each amplifier and their resulting transfer functions. Compare a simplified expression for the Laplace transfer function written in loop gain format. Figure 1 shows the internal block diagram of a VFB along with the internal model and closed loop transfer function. A(s) is the frequency-dependent open-loop gain of the op amp. It is modeled here as a single dominant pole response. Figure 1 uses an inverting gain configuration for comparison purposes because the FDA device is most clearly understood as a differential inverting configuration.
The transfer function has the desired gain in the numerator (-RF/RG) and a loop gain (LG) term in the denominator that determines the frequency response. One way to understand this LG is to plot [20 · Log (A(s))] and [20 · Log(1+RF/RG)] on the same grid. The key issues for this plot are: 1) the separation between these two curves at lower frequencies (this separation shows the magnitude of the loop gain); and 2) at what frequency they intersect.
At the point of intersection, sometimes called loop gain crossover, the term in the denominator of the transfer function drops to 1 + 1e-jθ (where jθ is the angle of that expression). The important check is that this angle is well away from -180 degrees to avoid closed loop oscillations or peaking. Figure 2 shows an example loop-gain plot of these two terms, where a simple single-pole response for A(s) is assumed and no phase added by the feedback network.
In Figure 2, the (1 + RF/RG) term is assumed to add no phase impact to the loop gain; only the open-loop phase of A(s) introduces loop phase shift in this simple example. This graph is the lower plot, where the loop gain crossover frequency is mapped down to find the remaining phase margin at that frequency.
Note that it is impossible for the VFB to change the signal gain without changing the loop-gain characteristic. This effect is where the gain bandwidth product (GBP) concept arises. If the gain increases, the bandwidth must decrease. If the gain decreases, the bandwidth increases, and the phase margin will normally decrease. (See Reference 1 for a more complete discussion.)
In contrast, the internal workings of a CFB op amp are quite different. Those blocks and the resulting closed loop transfer function for the inverting configuration are shown in Figure 3where, again, an inverting configuration is used and a single pole internal transimpedance gain is assumed for Z(s).
The CFB uses a unity-gain buffer across the two inputs. This forces the inverting node voltage to follow the non-inverting input voltage. That buffer is intended to present a low impedance to the inverting port, where a low level error current may be sensed and passed on to the output through a transimpedance gain. It is this internal transimpedance gain, Z(s), which acts in the same fashion as the VFB A(s) to provide a high DC gain with a dominant pole.
When the loop is closed, the same desired gain is achieved; but the loop-gain terms are very different. The CFB amplifier has a loop gain set by the forward transimpedance gain compared to the feedback impedance. Figure 4 plots the loop gain and phase for a typical CFB amplifier, where the feedback element is assumed to introduce no phase shift in this simplified analysis.
This plot looks very similar to the VFB plot except that the external element setting the loop gain is the feedback impedance alone. The greatest difference between the VFB and CFB amplifier is that the loop gain can be set separately from the signal gain using the feedback impedance. The feedback impedance becomes an independent compensation element, where the gain can then be set using the normal gain equations from whatever impedance value is selected for RF. This approach gives what is sometimes called gain bandwidth independence for the CFB amplifier. (See Reference 1 for a more detailed discussion.)
The final type of amplifier to be considered here is the new fully differential amplifier (FDA). Figure 5 shows the configuration and closed loop transfer function for this type of amplifier.
If the two feedback networks are allowed to be unmatched, the transfer function is fairly complicated. If they have a matched divider ratio (see Figure 5), the equations simplify to be the same as the inverting VFB transfer function. The effect of the separate common-mode loop is not shown. This loop acts to servo the average output voltage to a value set by a VOCMinput pin voltage (see Reference 2 for a more complete discussion of the FDA topology). For applications considered in this article, the FDA is treated as differential VFB device.
In the range of possible applications for wideband op amps, several types must use VFB devices. These circuits can sometimes be forced to work using CFB devices, but usually at the cost of complexity and poorer performance. Any circuit that requires flexibility in the feedback element and/or capacitors in the feedback will have stability problems with a CFB device. This instability is the result of the loop gain depending on the feedback impedance. Therefore, any circuit that needs a lot of flexibility in that impedance is going to interact with the achievable frequency response if a CFB amplifier is used.
The following example circuits should use a VFB device for implementation.
A. Transimpedance amplifiers. These circuits take a current source input, typically from a capacitive source, and turn it into a voltage at the output. The feedback resistor is the gain element and normally needs a compensation capacitor in parallel for correct operation. Figure 6 shows an example using the OPA657, a very wideband JFET input device uniquely suited to the transimpedance application. This device is a non-unity gain stable VFB with relatively low input noise voltage and very high gain bandwidth product. For a given diode source capacitance, the amplifier gain bandwidth product (GBP) determines the achievable bandwidth and/or transimpedance gain (Reference 3).
In this example, the 500 kΩ along with the 200 pF diode capacitance gives a noise gain zero at approximately 1.6 kHz. With the feedback capacitor set to achieve a maximally flat Butterworth response, the resulting F-3dB will be at the geometric mean of this zero and the 1.6 GHz gain bandwidth product of the OPA657. (Reference 3 gives a detailed analysis for compensation and noise in a transimpedance design.) Figure 7 shows the 1.6 MHz transimpedance bandwidth in a simulated frequency response of Figure 6.