This Table 1 will show representative, non-slew enhanced, single channel, unity gain stable VFA devices that offer both low offset voltage (<=1mV max) and lower input voltage noise (<4nV/√Hz). The next 3 tables will progress through different approaches to increase the slew rate. These low noise and offset devices are all bipolar. Low output offset will therefore require bias current cancellation in the design (ref.2). Done correctly, that will reduce the output DC error due to input bias currents to Rf*Ios. Note, once again, the highest speed devices require a non-RRO design. The reported gain of 1 SSBW shown here often exceeds the true Gain Bandwidth Product (GBP). For instance, the fastest 800MHz OPA820 actually shows a data sheet GBP of 240MHz. This is again due to phase margin at unity gain crossover for the LG being < 65deg extending the closed loop bandwidth considerably (a topic for a later blog).
To reduce this table of single channel VFA, the following were screened out –
- Slew Rate >400V/μsec
- Max Vio >1mV
- Input noise voltage > 4nV/√
- Rail-Rail Input (RRI) devices (those will appear in Table 4)
- 1k MSRP > $3.00 to get more active devices for new design.
- Obsolete Devices
The most common approach to delivering a VFA solution that offers higher slew rate is to decompensate the op amp. This limits their applicability to higher gain (or transimpedance) applications, but can deliver very good DC precision, normally lower input voltage noise, and a higher LSBW in a given supply current range. While there are many JFET or CMOS input decompensated options, most of those either have higher input noise or offset voltage. To show a few very low input bias current decompensated options, expand the screens to remove –
- Input voltage noise > 3.0nV/√Hz
- Max. input offset voltage > 2.5mV
- Price >$4.00
- Obsolete devices
The slew rates are generally higher using the de-compensated approach while most devices also show a lower input voltage noise and offset voltage. There are relatively few RRO or NRI devices where one of the most recent (OPA838, ref.7) does offer that combination with a slew enhanced input stage, very low power, low noise, device. Generally, these devices offer a very low input offset current allowing input bias current cancellation techniques to be applied (ref. 2). The descending SSBW*Gmin sort usually far exceeds the actual GBP for each device as the minimum operating closed loop gain has a low phase margin extending the closed loop bandwidth at that minimum recommended closed loop gain. Gmin is the minimum recommended gain. Usually set to hit some safe minimum phase margin – typically in the 30deg to 45deg region. That Gmin will usually be peaking the non-inverting SSBW, but not unstable. So, the “Minimum Stable Gain” is kind of a misnomer, where “Minimum Operating Gain” is more accurate. Inverting operation for a decompensated VFA can operate at any gain (including attenuation) using the simple compensation technique of ref. 8.
De-compensated high speed, single, VFA devices, in descending Gmin*SSBW (at Gmin)
Slew Enhanced, Unity Gain Stable, Voltage Feedback Amplifiers
The earliest approach to closing the gap between SSBW and LSBW in unity gain stable VFA op amps applied matched input buffer stages to a transconductance element. This shows an external VFA feedback characteristic but then emulates CFA slew rates as an increasing error voltage increases the available slewing current in that resistor sitting across the input buffers. An early example is shown in Figure 1, (page 10, ref.9) where the input stage and transconductance element are highlighted. At the onset of slew limiting, the input error voltage increases – increasing the available charging current through the current mirrors to the compensation capacitor, “C” in Figure 1.
VFA input stage emulating CFA slew enhancement
This approach can normally be identified by the relatively high input voltage noise that comes with it. Also, since the input buffers are not that matched, they usually show higher input offset voltage and currents. No offset current drift information is also a clue to this topology. Similar to the CFA amplifiers (that use similar input buffers), this approach to slew enhancement comes with generally poor DC precision and non-RRIO swing capability. It is sometimes difficult to identify this topology (although LTC provides Figure 1 in their datasheets for this type of part). Table 3 is a best effort extraction of a range of single channel, unity gain stable, very high slew rate devices similar to the design shown in Figure 1. Even though many of these devices have higher input noise and offset voltage, extreme values were cutoff in table 3 where parts excluded included (no screen on input offset voltage here) –
- En > 12nV/√Hz
- 1K MSRP > $3.00
- Non-unity gain stable devices
- Obsolete devices
While these devices essentially emulated the LSBW capability of CFA types, there is a lot of room for improvement in DC accuracy and noise. Also, similar to all CFA solutions, none of these can support swing to either rail on the inputs. The two sub 1mV offset LTC devices are silent on Ios drift in their datasheets.