SLOA332A July   2023  – September 2024 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. 1 Slew Rate Definition
    1. 1.1 Virtual Ground and Slew Rate
  5. 2 Bipolar Op Amp Slew Rate Example
  6. 3 CMOS 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. 4 Four Methods to Determine Boost or No Boost Using the Data Sheet
    1. 4.1 Method 1: Compare Slew Rate Versus Gain Bandwidth
    2. 4.2 Method 2: Compare Quiescent Current Versus Similar SR Devices
    3. 4.3 Method 3: Evaluate Large Signal Response
    4. 4.4 Method 4: Evaluate Small Signal Response
  8. 5 Slew Rate Dependencies on Circuit Signal Levels and Op Amp Gain Set by Feedback Network
  9. 6 How Much Output Slew Rate is Needed to Support a Sine Wave or Other Non-step Inputs
  10. 7 Stability Also Plays a Role in Observed Slew Rate
  11. 8 Summary
  12. 9 References
  13. 10Revision History

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.

TS321, TL074, TLV9001, OPA4991, OPA2991, OPA991, LMV831, OPA345, LMV821-N, OPA377-Q1, OPA376-Q1, OPA377, OPA376, TLV376, TLV9002 Simplified Bipolar Input Stage SchematicFigure 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. SR/SR[MAX]= I1-I2I1-I2

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.

TS321, TL074, TLV9001, OPA4991, OPA2991, OPA991, LMV831, OPA345, LMV821-N, OPA377-Q1, OPA376-Q1, OPA377, OPA376, TLV376, TLV9002 Bipolar SR/SR[max] Versus VIDFigure 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, 2mV (1mV*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 800mV to get the full slew rate.