The development of higher speed amplifiers, from a DC precision perspective, has been one of improving that precision while also providing higher slew rates - which ties to available Large Signal BandWidth (LSBW). The highest slew rates intrinsic to the Current Feedback Amplifier (CFA) approach come with relatively poor DC precision. Enhancing the available slew rate in the Voltage Feedback Amplifiers (VFA) has progressed through a number of approaches that will be shown here. The Fully Differential Amplifier (FDA) offers both options where the CFA-based versions provide very high slew rates, with poor DC precision, while VFA based FDAs have progressed using similar techniques to improve precision with higher slew rates as the core VFAs. The FDA brings a number of new DC precision considerations due to the added common mode control loop (ref. 1). This part 3 will progress through the different VFA approaches with Part 4 moving on to CFA and FDA DC precision considerations.
Paths to Increasing Slew Rate in VFA Op Amps
Achievable DC precision (and noise) is largely an input stage design issue. The VFA brings some natural advantages to lower DC errors. Recall, the first level DC errors for the VFA are the three input error terms. Using these to calculate the output error has been covered by many earlier references (ref. 2).
- Input offset voltage and drift
- Input bias currents and drift
- Input offset current and drift
Occasionally, some other error terms are lumped in with the input offset voltage (ref. 3). While it is certainly true the physical input error voltage will have a Vout/Aol term, since that term is output signal level dependent, it is properly captured by a gain error to the output. The input error voltage due to open loop gain (Aol) is not added to the static DC input offset voltage. It is instead accounted for by the gain compression term described by LG/(LG+1) where LG = Aol/NG, NG = Noise Gain. So, what is the input offset voltage when the VFA op amp is operating at unity gain, split balanced supplies, with a grounded non-inverting input? Since the output voltage will be operating very near ground, internal to the input differential stage, there is an error voltage that should be very nearly 0V. The observed external offset voltage is the required compensating voltage for input stage imbalances to get the actual error voltage internally to zero.
As classic VFA Gain Bandwidth Products (GBP) steadily increased through the early 1980s, their slew rates did not keep pace where a typical unity gain stable, high-speed, Harris op amp (ref. 4) would have a Small Signal BandWidth (SSBW) far exceeding the available LSBW. The two dominant approaches to increased LSBW in VFA devices through about 2004 were to use an input stage that emulates CFA operation or to provide de-compensated amplifiers. Non-unity gain stable, de-compensated, VFA devices reduce the compensation capacitor value and often increase the input stage gm (lower degeneration R in the input stage giving lower input noise as well). Hence, for the same approximate slewing current available at the compensation cap, more slew rate can be made available. Improving DC precision and noise while increasing slew rate is considered in the context of lowering supply current as well. It is always possible to increase slew rate and reduce input voltage noise with more quiescent current. Delivering solutions that improve both at ever lower quiescent current is implicit in this discussion.
First, let’s review unity gain stable, non-slew enhanced, high speed (>20MHz) VFA options. Slew enhanced unity gain stable VFA can be sorted out by ranking a descending estimate of LSBW/Icc. Those will appear in later tables, but for now isolate down to modest slew rate, lower noise, precision, unity gain stable VFA. As described in ref. 5, many of the more recent VFA introductions include Rail-to-Rail Output (RRO) along with Negative Rail Input (NRI) or Rail-to-Rail (RRI). Some of the lower noise devices are non-RRIO designs. The RRI devices include either a x-over network or internal charge pump to bias the input stage beyond the supplies applied (ref. 6). To show better DC offset devices over the total useable input range, RRI devices will be excluded until Table 4. Unity gain stable VFA devices long struggled to deliver low input voltage noise where several recent releases seem to have broken through that limit – initially showing higher supply current or much lower slew rate but with improved solutions included in Table 1. There are also many low offset, higher speed, unity-gain stable CMOS or JFET input devices. Those generally show higher (>4nV/√Hz) input voltage noise and are not as power efficient in their LSBW/Icc ratio. They will have the benefit of removing input bias and offset current errors in the output DC offset calculation.