AD8051/AD8052/AD8054
Overdrive Recovery
Overdrive of an amplifier occurs when the output and/or input
range are exceeded. The amplifier must recover from this over-
drive condition. As shown in Figure 36, the AD8051/AD8052/
AD8054 recovers within 60 ns from negative overdrive and
within 45 ns from positive overdrive.
2.60
2.55
2.50
2.45
2.40
Figure 38. AD8051/AD8052 200 mV Step Response:
C
L
= 50 pF
10000
V
S
= +5V
30%
OVERSHOOT
P
F
Figure 36. Overdrive Recovery
Driving Capacitive Loads
R
S
= 3
1000
R
S
= 0
Consider the AD8051/AD8052 in a closed-loop gain of +1 with
+V
S
= 5 V and a load of 2 kΩ in parallel with 50 pF. Figures 37
and 38 show its frequency and time domain responses, respec-
tively, to a small-signal excitation. The capacitive load drive of
the AD8051/AD8052/AD8054 can be increased by adding a
low valued resistor in series with the load. Figures 39 and 40
show the effect of a series resistor on capacitive drive for varying
voltage gains. As the closed-loop gain is increased, the larger
phase margin allows for larger capacitive loads with less peak-
ing. Adding a series resistor with lower closed-loop gains ac-
complishes the same effect. For large capacitive loads, the
frequency response of the amplifier will be dominated by the
roll-off of the series resistor and the load capacitance.
8
6
4
2
0
2
4
6
8
10
0.1
V
S
= +5V
G = +1
R
L
= 2k
C
L
= 50pF
V
O
= 200mV p-p
500
CAPACITIVE LOAD
100
R
G
10
V
IN
100mV STEP
50
R
F
R
S
V
OUT
C
L
1
1
2
3
4
A
C L
– V/V
5
6
Figure 39. AD8051/AD8052 Capacitive Load Drive vs.
Closed-Loop Gain
1000
V
S
= +5V
30%
OVERSHOOT
CAPACITIVE LOAD – pF
R
S
= 10
GAIN – dB
R
S
= 0
100
R
G
V
IN
100mV STEP
50
R
F
R
S
V
OUT
C
L
1
10
FREQUENCY – MHz
100
10
1
2
3
4
A
C L
– V/V
5
6
Figure 37. AD8051/AD8052 Closed-Loop Frequency
Response: C
L
= 50 pF
Figure 40. AD8054 Capacitive Load Drive vs. Closed-Loop
Gain
Circuit Description
The AD8051/AD8052/AD8054 is fabricated on Analog Devices’
proprietary eXtra-Fast Complementary Bipolar (XFCB) pro-
cess, which enables the construction of PNP and NPN transis-
tors with similar f
T
s in the 2 GHz–4 GHz region. The process is
dielectrically isolated to eliminate the parasitic and latch-up
–12–
REV. B
AD [ ANALOG DEVICES ]
