Datasheet
ADA4895-1/ADA4895-2 Data Sheet
Rev. A | Page 20 of 24
HIGH GAIN BANDWIDTH APPLICATION
The circuit in Figure 52 shows cascaded dual amplifier stages
using the ADA4895-1/ADA4895-2. Each stage has a gain of +10
(20 dB), making the output 100 times (40 dB) the input. The total
gain bandwidth product is approximately 9 GHz with the device
operating on 6 mA of quiescent current (3 mA per amplifier).
V
OUT
V
IN
R
T
50Ω
R
L
1kΩ
R
F
226Ω
C
F
2pF
C1
5pF
R
G
25.5Ω
R
F
226Ω
C
F
2pF
R
G
25.5Ω
R1
249Ω
10186-048
Figure 52. Cascaded Amplifier Stages for High Gain Applications (G = +100)
Figure 53 shows the large signal frequency response for two cases.
The first case is with installed feedback capacitors (C
F
= 2 pF), and
the second case is without these capacitors. Removing the 2 pF
feedback capacitors from this circuit increases the bandwidth,
but adds about 0.5 dB of peaking.
0
4
8
12
16
20
24
28
32
36
40
44
0.1 1 10 100 1000
GAIN (dB)
FREQUENCY (MHz)
C
F
= 2pF
NO C
F
V
OUT
= 2V p-p
G = +100
10186-049
Figure 53. Large Signal Frequency Response, G = +100, V
S
= ±5 V
To better balance the second stage and remove the current offset
contribution, an R1C1 circuit can be sized to correct for any
mis-match between the source impedance and the feedback
network impedance on the input amplifier. (In the example
shown in Figure 52, R1 = 249 Ω and C1 = 5 pF.) The offset of
each amplifier is within the same statistical range. As configured,
the offset of the output amplifier is not statistically significant to
the overall offset of the system.
Figure 53 was captured using a ±5 V supply; however, this circuit
also operates with supplies from ±1.5 V to ±5 V as long as the
input and output headroom values are not violated.
FEEDBACK CAPACITOR APPLICATION
For applications where frequency response flatness is necessary,
or a larger feedback resistor value is desired, a small feedback
capacitor in parallel with the feedback resistor can be used to
reduce peaking and increase flatness.
Figure 54 shows the small signal frequency response with and
without a feedback capacitor.
–15
–12
–9
–6
–3
0
3
6
1 10 100 1000
NORMALIZED CLOSED-LOOP GAIN (dB)
FREQUENCY (MHz)
R
F
= 449Ω
R
F
= 1kΩ
R
F
= 1kΩ, C
F
= 0.5pF
R
F
= 499Ω, C
F
= 1pF
V
S
= ±2.5V
V
OUT
= 200mV p-p
R
L
= 1kΩ
G = +10
ADA4895-1 SOIC
10186-153
Figure 54. Small Signal Frequency Response With and Without a Feedback
Capacitor