SLOA332 july   2023 LMV821-N , LMV831 , OPA2991 , OPA345 , OPA376 , OPA376-Q1 , OPA377 , OPA377-Q1 , OPA4991 , OPA991 , TL074 , TLV376 , TLV9001 , TLV9002 , TS321

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Slew Rate Definition
    1. 1.1 Virtual Ground and Slew Rate
  5. 2Bipolar Op Amp Slew Rate Example
  6. 3CMOS Op Amp Slew Rate Example
    1. 3.1 Slew Boost Example 1
    2. 3.2 Slew Boost Example 2
    3. 3.3 Slew Boost Summary
  7. 4Four Methods to Determine Boost or No Boost Using the Data Sheet
  8. 5Slew Rate Dependencies on Circuit Signal Levels and Op Amp Gain Set by Feedback Network
  9. 6How Much Output Slew Rate is Needed to Support a Sine Wave or Other Non-step Inputs
  10. 7Stability Also Plays a Role in Observed Slew Rate
  11. 8Summary
  12. 9References

2 Bipolar Op Amp Slew Rate Example

Many bipolar op amp input stages simplify to the circuit in Figure 2-1. The VID, [IN+]-[IN-] voltage, controls how the bias current (B) is split between current paths I1 and I2. Current I1 is mirrored 1:1, to create an output current, I2-I1, that can vary from -B to +B. This output current charges the compensation capacitor (CC) and this charge rate is inverted to become the output slew rate.

GUID-AD2646B8-8D8E-4826-8155-A3276A442C1C-low.svg Figure 2-1 Simplified Bipolar Input Stage Schematic

The output slew rate (SR) can be varied from -B/CC to +B/CC. The result of B/CC is the slew rate that is specified on the data sheet. For some op amps, the positive and negative slew rate can be a little different; in this example the slower rate is recorded. The SR in the data sheet is always the magnitude of SR, ignoring polarity.

Current B and capacitance CC are different for every bi-polar op amp. However, the relationship between VID and SR / max SR ratio is similar for most. This consistent relationship is based on two formulas, the first shown in Equation 1 where k = Boltzmann’s constant, T = Temperature (Kelvin), q = Electron charge. The second formula, Equation 2, is the percentage of full slew rate.

Equation 1. VID=k×Tq×ln⁡I1I2
Equation 2. S R / S R [ M A X ] =   I 1 - I 2 I 1 - I 2

Zero slew rate (0%) occurs when I1=I2 at VID=0 (more accurately stated as VID = -VOS). Maximum slew rate (100%) occurs when one of the currents [I1 and I2] are zero and the other is at full current. This requires |VID| >> 100mV. Figure 2-2 shows the relationship between VID and slew rate relative to maximum slew rate for most bi-polar op amps.

GUID-20230509-SS0I-5Q9K-PWHL-HRGSG0QXF8GQ-low.svg Figure 2-2 Bipolar SR/SR[max] Versus VID

Bipolar SR/SR[max] versus VID chart also applies to Darlington bipolar input stages that have current sources on all emitters. There are a few Darlington op amps without a current source on the first emitter. TS321 is an example of a device that does not have a current source on the first emitter. For the TS321 op amp, VID needs to be double the chart value. For example, 2 mV (1 mV*2) gives 2% of the full slew rate. For JFET devices like the TL074, the VID needs to be eight times larger, as it takes 800 mV to get the full slew rate.