1. OPA2227 All NPN Output Stage Analysis
Precision Linear Analog Applications
April 19th, 2018 – Tim Green, Thomas Kuehl

2. Summary
Note: The All NPN Output Stage Analysis presented in this presentation is NOT an exact model for the OPA2227 but rather a similar architecture to show the cause of high frequency oscillations on an All NPN Output Stage due to capacitive loading and the use of an output R-C snubber to fix the issue.
An R-C snubber is recommended to prevent high frequency oscillations (>80Mhz) due to light capacitive loads on All NPN Output Stage.
Impedance analysis for stability is presented.
The all NPN output stage is used in the OPA The Composite PNP/NPN transistor is used for sinking current in this topology.
The all NPN output stage is analyzed for output impedance.
The Composite PNP/NPN transistor stage which sinks current is shown to have an inductive output impedance, whereas the NPN output stage which sources current is shown to be resistive.
Light cap loading of the Composite PNP/NPN transistor can cause oscillation. This is verified in simulation.
An output R-C snubber network can eliminate oscillations due to capacitive loading of the Composite PNP/NPN transistor stage. Analysis is provided to prove this.
Note: All TINA-TI simulations can be run on the embedded schematics in this presentation by downloading the free TI SPICE simulator, TINA-TI, at:

3. Recommended Snubber for OPA2227 to prevent Light Cload oscillations

4. Impedance Analysis for Stability

5. Typical L-C Circuit

6. Typical L-C Circuit ImpedancesXL
L=1uH
Frequency (161kHz) where XC = XL for L-C circuit expect resonance

7. L-C Resonance from Transient Test

8. L-C Resonance from Transient Test
Transient Resonance at Frequency (161kHz) where XC = XL predicted L-C circuit resonance

9. Technical Reference: Closed Loop Impedance Stability Analysis (embedded pdf document)

10. OPA2227 All NPN Output Stage Analysis

11. OPA2227 Output stage lower transistor design
Traditional PNP output transistor has gain and bandwidth limitations when compared to NPN
Composite transistor PNP/NPN is used in the Output Stage to enhance current sink performance equivalent to upper NPN source transistor
The structure of the composite transistor has local feedback
Composite Transistor PNP/NPN

12. All NPN Output Stage Architecture

13. All NPN Output Stage Architecture Zout (Closed loop Output Impedance)
Note how Zout_plus looks resistive and capacitive, whereas Zout_minus (composite transistor) looks inductive as frequency increases. Specific capacitive loads will cause composite transistor oscillations from any disturbance (noise, transient spikes, etc.)

14. All NPN Output Stage Architecture – Light Cload

15. All NPN Output Stage Architecture – Light Cload

16. All NPN Output Stage Architecture – Light Cload

17. All NPN Output Stage Architecture – Light Cload
fres from Transient Analysis is 10.53MHz and fres predicted form first look at AC Analysis was 12.7MHz. AC first look assumes pure inductance whereas actual resonant circuit has resistive and capacitive effects of Zout.

18. Snubber minimizes Cload effects of Composite PNP/ NPN
Rsn and Csn form a snubber which prevent light Cload from causing PNP/NPN composite stage from oscillating. Rsn limits output current into Cload during fast transitions of output stage. Also in real op amp output Zout and Csn form a pole in the higher frequency portion of the Aol and Rsn and Csn add a zero into this region so phase margin is not degraded

19. Snubber minimizes Cload effects of Composite PNP/ NPN

20. Snubber minimizes Cload effects of Composite PNP/ NPN

21. Snubber minimizes Cload effects of Composite PNP/ NPN