Editor’s note: OK designers, pay attention to this series of tutorials for an in-depth understanding of these really useful amplifier architectures that will help you in your designs with a solid and insightful knowledge gained by these articles from the engineer I consider to be the foremost expert on high speed Current Feedback, Voltage Feedback and Fully Differential Amplifiers amplifiers (This is from my first-hand knowledge of working with him for many years in the field). File these articles away in your treasure chest of tech reference articles.
The initial edition in this series (ref. 1) explored the I/O range options for high speed Voltage Feedback Amplifier” (VFA) op amps. Here, we will pick up those same issues for the remaining two most popular high-speed amplifier types. First the “Current Feedback Amplifier” (CFA) and then the “Fully Differential Amplifier” (FDA). The I/O range options and considerations for the CFA will be pretty straightforward while the FDA brings a number of new I/O range issues and capabilities. Those will be shown here along with some of the application issues driving these device definitions with detailed parameters on a representative selection of both types included.
Input/Output Range issues for Current Feedback Amplifiers (CFAs)
The first commercially available CFAs appeared from Comlinear Corp. in the early 1980s as hybrid devices. The topology benefits greatly from symmetric NPN and PNP devices where those (vertical PNPs) first started to show up in IC processes in the mid to late 1980s. The first monolithic CFA from Comlinear (CLC400, ref. 2a) appeared in Oct. 1988 – within months of two earlier monolithic CFA introductions - the AD811 (ref. 2b) and EL2020(ref.2c). Even with 30 years of new CFA introductions, there are “NO” devices that include either supply rail in the input range.
The nature of the CFA requires a unity gain buffer across the V+ to V- pins in the typical op amp drawing. However, that V- (In- in Fig. 1) pin is also required to pass a bipolar error current up and down through current mirrors to each supply. Getting that cascoded error current (hence, “current feedback”) through the emitter followers at the V- node up and down to current mirrors off the supplies requires some headroom to the supplies for this unity gain buffer from the V+ input to V- node. A typical circuit diagram highlighting this very simple input buffer is shown in Figure 1 (ref. 2c). While this earliest device optimistically included offset balance pins (a carryover from the VFA IC designs), that external balance adjustment does nothing to improve output offset drift and rarely appeared in later CFA devices.
Circuit diagram for early CFA monolithic op amps (EL2020)
Similarly, while there are a few Rail-to-Rail Output (RRO) CFA devices (ref. 3), all other CFAs are non-RRO. The input buffer needs more headroom than most output stages. This has the effect of showing product choices with less headroom on the output vs. the input pins (and symmetric to each supply on both I/O). By far the dominant applications for CFA provide some non-inverting gain (or run inverting), so this increased headroom on the inputs is not a limiting factor for the output swing.
While all CFA devices are intrinsically unity gain stable (ref.4), since many of the applications are at some higher non-inverting gain, many devices are specified (and characterized) at gains of +2V/V and sometimes higher. In theory, the AC performance can be held constant for CFAs. However, the lower voltage swings required across the input buffer operating at higher non-inverting gains (or inverting where there is no voltage swing across the input buffer) will move the Full Power BandWidth (FPBW) limit to the main amplifier instead of the open loop buffer across the inputs.
Representative CFA devices grouped by application space.
So where would you find a CFA solution vs. the more prevalent VFA solutions? Very generally speaking, while VFA devices are commonly found leading up to ADCs, going away from a DAC to provide a signal generator function is dominated by CFA solutions. These offer significant large signal linearity vs. quiescent power advantages over VFAs due to their “slew rate on demand” capability. Many early CFA applications were simple video line drivers at a gain of +2V/V to recover the backmatch loss for video coax transmission. At the same time, the original genealogy for the CFA amplifier came from the HP Arbitrary Waveform Generator (AWG) output stages, and many signal generators continue to use a CFA device right behind that output BNC or SMA connector.
The largest opportunity for dual CFAs came with the emergence in the mid-1990s of the various twisted pair line drivers for the xDigital Subscriber Line (xDSL) domain. This has driven many product developments yielding general purpose dual CFA devices. Most recent developments for wireline drivers are specialized and only suitable to differential driver applications (ref. 5) - as opposed to 2 independent op amps. The core CFA remains ubiquitous in the various wireline communications - whether it be xDSL, PLC, G.Fast, Smartgrid, etc as the differential output driver. Dual coupled CFAs are often called “ports” in the datasheets to indicate one committed pair of push/pull CFA drivers.
Higher output current CFA solutions are usually found in AWG and wireline communications. This first CFA table will screen to >200mA linear output current and include both singles (AWG) and duals (xDSL). Some dual CFA line drivers do not even offer a single channel version (ref. 6). Also, some dual CFAs can only be used as differential push/pull output stages (ref. 7) where those will be excluded from Table 1. The dual CFAs shown here can be paralleled for a load sharing configuration or used independently for single ended line drivers.
To get newer, high output current CFAs, Table 1 also screened out:
- Output Headroom >2V
- If separate part numbers for disable and non-disable, only the disable version listed
- Input voltage noise < 4nV/√Hz.
- Fixed internal gain CFA’s.
- Obsolete devices
By far the fastest, high output current, CFA is the recently introduced THS3491 (ref. 8) – intended for AWG outputs. Note the generally higher max. Vs here to support higher output voltage swings for AWG or wireline drivers.
The range of CFA devices intended for lower output power is much wider including many single/dual/quad families. Again, all of these devices are also non-RRIO where usually a bit more headroom is required on the inputs vs the output. To reduce this table to a representative listing, only singles are shown where the same screens as Table 1 were used with only devices offering <200mA linear output current and a 1k price <$3.00 to get more active devices for new designs.
These are all permutations on the original CFA design of Figure 1, with one device (OPA684, ref. 9) applying a closed loop input buffer to provide more constant bandwidth over gain setting in a low power device. Using a closed loop input buffer stage reduces the open loop impedance looking into the V- input reducing that parasitic effect in the loop gain equation (ref. 4). There is a lot of similarity in the max operating supply voltage for the devices in Table 2 - indicating the complementary bipolar process technologies used are similar. There are no CMOS CFAs as can be seen looking at the Ib max column. These V+ input bias currents are always the cancellation of two bias currents. The V- input bias current is usually higher and un-related to the V+ bias current precluding bias current cancellation techniques to improve output offsets.