"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

AAVD VD – Large-Signal Differential
Voltage Amplification
LARGE-SIGNAL DIFFERENTIAL VOLTAGE
AMPLIFICATION AND PHASE SHIFT
vs
FREQUENCY
106
105
104
103
102
101
1
1
Phase Shift
Differential
Voltage
Amplification
0°
45°
90°
135°
10 100 1 k
180°
10 k 100 k 1 M 10 M
f – Frequency – Hz
VCC ± = 5 V to VCC ± = 15 V
RL = 2 kΩ TA = 25°C
Phase Shift
VVO O – Output Voltage – mV
28
24
20
16
12
8
4
0
–
4
ÁÁ
ÁÁ
Figure 7–4. TL07X Frequency and Time Response Plots
Voltage-Feedback Op Amp Compensation
tr
Internal Compensation
OUTPUT VOLTAGE
vs
ELAPSED TIME
Overshoot
0 0.1 0.2 0.3 0.4 0.5 0.6
t – Elapsed Time – µs
VCC ± = 15 V RL = 10 kΩ
TA = 25°C
The TL07X pulse response plot shows approximately 20% overshoot. There is no loading
capacitance indicated on the plot to account for a seemingly unconditionally stable op
amp exhibiting this large an overshoot. Something is wrong here: the analysis is wrong,
the plots are wrong, or the parameters are wrong. Figure 7–5 shows the plots for the
TL08X family of op amps, which are sisters to the TL07X family. The gain/phase curve
and pulse response is virtually identical, but the pulse response lists a 100 pF loading capacitor.
This little exercise illustrates three valuable points: first, if the data seems wrong
it probably is wrong, second, even the factory people make mistakes, and third, the loading
capacitor makes op amps ring, overshoot, or oscillate.
90%
10%
0.7
7-5