AN2653 Datasheet, PDF (19/22 Pages) STMicroelectronics – Operational amplifier stability compensation methods

AN2653
Snubber network compensation method
Table 10. Results of snubber network compensation for different load conditions in the case of
a gain configuration of either -30 or +31 (R_g = 100 Ω and R_f = 3 kΩ) for TS507
R_L = 1 kΩ
R_L = 10 kΩ
R_L = 100 kΩ
C_L
R_SN
C_SN Mg⁽¹⁾ Mϕ⁽¹⁾ R_SN
C_SN Mg⁽¹⁾ Mϕ⁽¹⁾ R_SN
C_SN Mg⁽¹⁾ Mϕ⁽¹⁾
(Ω)
(nF) (dB) (degree) (Ω)
(nF) (dB) (degree) (Ω)
(nF) (dB) (degree)
26 88.2 °
25.2 88.4 °
25.1 88.4 °
1 nF
198 100 29.8 87.3 ° 168.5 100 29.8 87.3 ° 166 100 29.8 87.3 °
7.9 87.9 °
10 nF
30.7 100 19.6 82.7 ° 30
7.2 88 °
100 19.2 82.7 ° 30
7.1 88.1 °
100 19.1 82.8 °
-4.2 -44.7 °
-4.5 -45.8 °
-4.5 -45.9 °
100 nF
16.6 100 3.4 63.8 ° 16.5 100 3.3 64 ° 16.5 100 3.3 64.1 °
1. Negative values indicate instability.
The results are almost the same whether R_L = 1 kΩ, 10 kΩ or 100 kΩ because in all cases, at
the frequency range that has a significant impact on stability, the resistive load on the
amplifier is R_SN // R_L ≅ R_SN.
From Table 7, you can see that, in the case of a voltage follower configuration, this
compensation method doesn’t work for capacitive loads higher than 1 nF because C_SN is
limited to 100 nF. Figure 27 and Figure 28 show that lower R_SN values would give better
results. However, in this case, these abacus are not valid because the C_SN impedance is not
negligible compared with R_SN at the frequency range where R_SN plays its role of stabilizer.
For R_L = 10 kΩ and C_L = 1 nF, the snubber network compensation method gives the loop
gain and closed loop gain shown in Figure 31 and Figure 32 respectively.
Figure 31. Loop gain
Voltage Follower Configuration - Loop Gain
150
130
110
90
Compensation with the Snubber Network Technique
TS507 :
Vcc = 5 V
Vicm = 2,5 V
T = 25 °C
RL = 10 kΩ
CL = 1 nF
0
-30
-60
-90
70
-120
50
-150
30
-180
10
-210
-10
-240
-30
-270
Figure 32. Measured closed loop gain
Voltage Follower Configuration - Closed Loop Gain
Compensation with the Snubber Network Technique
20
TS507 :
Vcc = 5 V
10
Vicm = 2,5 V
T = 25 °C
0
RL = 10 kΩ
CL = 1 nF
-10
With Snubber Network Compensation Technique
-20
-30
Without Compensation
-40
Gain without Compensation
Phase without Compensation
Frequency (Hz)
Gain with RSN = 30 Ω and CSN = 100 nF
Phase with RSN = 30 Ω and CSN = 100 nF
Frequency (Hz)
Gain without Compensation Gain with RSN = 30 Ω and CSN = 100 nF
This compensation has the main advantage of neither reducing the output swing nor the
gain accuracy, unlike the two other compensation methods. Nevertheless, for the TS507,
this method is limited to capacitive loads lower than 1 nF in a voltage follower configuration,
and the stability improvement provided by the compensation is not as good as with the two
other methods.
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