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LT1206

1206fa

APPLICATIONS INFORMATION

is ﬂat to 0.35dB to 30MHz. The network has the greatest

effect for CL in the range of 0pF to 1000pF. The graph of

Maximum Capacitive Load vs Feedback Resistor can be

used to select the appropriate value of feedback resistor.

The values shown are for 0.5dB and 5dB peaking at a gain

of 2 with no resistive load. This is a worst case condition,

as the ampliﬁer is more stable at higher gains and with

some resistive load in parallel with the capacitance. Also

shown is the – 3dB bandwidth with the suggested feedback

resistor vs the load capacitance.

Although the optional compensation works well with ca-

pacitive loads, it simply reduces the bandwidth when it is

connected with resistive loads. For instance, with a 30Ω

load, the bandwidth drops from 55MHz to 35MHz when the

compensation is connected. Hence, the compensation was

made optional. To disconnect the optional compensation,

leave the COMP pin open.

Shutdown/Current Set

If the shutdown feature is not used, the SHUTDOWN pin

must be connected to ground or V –.

The shutdown pin can be used to either turn off the bias-

ing for the ampliﬁer, reducing the quiescent current to

less than 200µA, or to control the quiescent current in

normal operation.

The total bias current in the LT1206 is controlled by the cur-

rent ﬂowing out of the shutdown pin. When the shutdown

pin is open or driven to the positive supply, the part is shut

down. In the shutdown mode, the output looks like a 40pF

capacitor and the supply current is typically 100µA. The

shutdown pin is referenced to the positive supply through

an internal bias circuit (see the simpliﬁed schematic). An

easy way to force shutdown is to use open drain (collec-

tor) logic. The circuit shown in Figure 2 uses a 74C904

buffer to interface between 5V logic and the LT1206. The

switching time between the active and shutdown states

is less than 1µs. A 24k pull-up resistor speeds up the

turn-off time and insures that the LT1206 is completely

turned off. Because the pin is referenced to the positive

supply, the logic used should have a breakdown voltage

of greater than the positive supply voltage. No other

circuitry is necessary as the internal circuit limits the

shutdown pin current to about 500µA. Figure 3 shows

the resulting waveforms.

FREQUENCY (MHz)

–8

–4

100

LT1206 • F01

–6

–2

VS = ±15V

RF = 1.2k

COMPENSATION

RF = 2k

NO COMPENSATION

RF = 2k

COMPENSATION

Figure 1

–

LT1206

S/D

15V

–15V

VIN

24k

ENABLE

VOUT

LT1206 • F02

15V

74C906

VOUT

ENABLE

AV = 1

RF = 825Ω

RL = 50Ω

RPU = 24k

VIN = 1VP-P

µs/DIV

1206 F03

Figure 2. Shutdown Interface

Figure 3. Shutdown Operation

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