SBOA356 August   2020 OPA1656 , OPA210 , OPA2210 , OPA2320 , OPA2320-Q1 , OPA320 , OPA320-Q1

 

  1.   Trademarks
  2. 1 Introduction
  3. 2 Voltage Offset
  4. 3 Bandwidth
  5. 4 Cascaded Amplifier Bandwidth
  6. 5 Cascaded Amplifier Offset
  7. 6 Multi-Stage Amplifiers
  8. 7 Normal Distributions in Offset Voltage
  9. 8 Noise Considerations
  10. 9 Summary
  11. 10Resources
    1. 10.1 TI Recommended Parts
    2. 10.2 TI Precision Labs Training Videos
    3. 10.3 TI Recommended Resources

6 Multi-Stage Amplifiers

Equation 3 can be expanded for any number of gain stages, i.e. the sum of the offsets of each stage multiplied by the gain of every stage succeeding them.

When comparing between different op amps or circuit layouts, it is often helpful to divide Voso by the total gain of the system to obtain the input-referred offset. Table 6-1 shows simulation results for a 1000 V/V amplifier circuit from one to four gain stages. Each stage employs the OPA2210 dual channel, low-noise, low-offset, rail-to-rail output, precision amplifier.

Table 6-1 Bandwidth and offset for multi-stage amplifiers

1 Stage

2 Stage

3 Stage

4 Stage

Gain/Stage (V/V)

1000

31.6

10

5.62

Input-Referred Offset (µV)(1)

5.02

5.17

5.59

6.17

Fc(2) (Hz)18k364k957k1530k
(1) The typical value for Vosi (+5µV) was used for each stage. This is common for spice simulations, but as it turns out the polarity of each stage’s offset can be an important factor.
(2) The typical value for gain bandwidth (18MHz) was used for simulation. In practice the bandwidth of an amplifier can vary from the typical value.

We can see that there is an inherent tradeoff between bandwidth, offset voltage, and the number of gain stages. For a low-offset precision op amp such as the OPA2210, the increase in offset voltage is minimal compared to the increase in bandwidth. This highlights the importance of using precision op amps, especially in the early stages of an amplifier circuit.