LT1229 Linear Technology, LT1229 Datasheet - Page 7

• Manufacturer Part Number: LT1229
• Description: Dual and Quad 100MHz Current Feedback Amplifiers
• Manufacturer: Linear Technology

A
The LT1229/LT1230 are very fast dual and quad current
feedback amplifiers. Because they are current feedback
amplifiers, they maintain their wide bandwidth over a wide
range of voltage gains. These amplifiers are designed to
drive low impedance loads such as cables with excellent
linearity at high frequencies.
Feedback Resistor Selection
The small-signal bandwidth of the LT1229/LT1230 is set
by the external feedback resistors and the internal junction
capacitors. As a result, the bandwidth is a function of the
supply voltage, the value of the feedback resistor, the
closed-loop gain and load resistor. The characteristic
curves of Bandwidth versus Supply Voltage are done with
a heavy load (100 ) and a light load (1k) to show the effect
of loading. These graphs also show the family of curves
that result from various values of the feedback resistor.
These curves use a solid line when the response has less
than 0.5dB of peaking and a dashed line when the re-
sponse has 0.5dB to 5dB of peaking. The curves stop
where the response has more than 5dB of peaking.
At a gain of two, on 15V supplies with a 750 feedback
resistor, the bandwidth into a light load is over 160MHz
without peaking, but into a heavy load the bandwidth
reduces to 100MHz. The loading has so much effect
because there is a mild resonance in the output stage that
enhances the bandwidth at light loads but has its Q
reduced by the heavy load. This enhancement is only
useful at low gain settings; at a gain of ten it does not boost
the bandwidth. At unity gain, the enhancement is so
effective the value of the feedback resistor has very little
effect. At very high closed-loop gains, the bandwidth is
PPLICATI
R
F
= R
G
Small-Signal Rise Time with
O
= 750 , V
U
S
S
I FOR ATIO
U
= 15V, and R
W
LT1229 • TA04
L
= 100
U
limited by the gain bandwidth product of about 1GHz. The
curves show that the bandwidth at a closed-loop gain of
100 is 10MHz, only one tenth what it is at a gain of two.
Capacitance on the Inverting Input
Current feedback amplifiers want resistive feedback from
the output to the inverting input for stable operation. Take
care to minimize the stray capacitance between the output
and the inverting input. Capacitance on the inverting input
to ground will cause peaking in the frequency response
(and overshoot in the transient response), but it does not
degrade the stability of the amplifier. The amount of
capacitance that is necessary to cause peaking is a func-
tion of the closed-loop gain taken. The higher the gain, the
more capacitance is required to cause peaking. We can
add capacitance from the inverting input to ground to
increase the bandwidth in high gain applications. For
example, in this gain of 100 application, the bandwidth can
be increased from 10MHz to 17MHz by adding a 2200pF
capacitor.
Boosting Bandwidth of High Gain Amplifier with
49
46
43
40
37
34
31
28
25
22
19
C
G
1
Capacitance on Inverting Input
V
IN
R
5.1
G
–
+
FREQUENCY (MHz)
LT1229
1/2
510
R
C
10
F
G
C
= 0
G
LT1229/LT1230
= 4700pF
C
G
= 2200pF
LT1229 • TA06
LT1229 • TA05
V
100
OUT
7