APPLICATION NOTES

CURRENT LIMIT

Figure 2 is an active short circuit protection scheme for the

MSK 611. The following formula may be used for setting cur-

rent limit:

Current Limit ≈ 0.6V / Rsc

as follows:

This formula guarantees that Q2 and Q4 will always have suf-

ficient base current to be in operation. This circuit can be made

tolerant of high frequency output current spikes with the addi-

limit is unnecessary, short pin 7 to pin 8 and pin 11 to pin 12.

Pin 8 can be tied to ground if swing through zero is not desired.

402Ω

Ω

Ω

Ω

Ω

301Ω

Ω

Ω

Ω

Ω

100Ω

Ω

Ω

Ω

Ω

-10V/V

-20V/V

-50V/V

0.25-2pF

N/A

N/A

4.02KΩ

Ω

Ω

Ω

Ω

6.04KΩ

Ω

Ω

Ω

Ω

5KΩ

Ω

Ω

Ω

Ω

VOLTAGE

GAIN

Table 1

FEED FORWARD TOPOLOGY

The MSK 611 employs a circuit topology known as "feed

forward". This inverting configuration allows the user to real-

ize the excellent D.C. input characteristics of a differential am-

plifier without losing system bandwidth. The incoming signal

is split at the input into its A.C. and D.C. component. The D.C.

component is allowed to run through the differential amplifier

where any common mode noise is rejected. The A.C. compo-

nent is "fed forward" to the output section through a very high

speed linear amplifier where it is mixed back together with the

D.C. component. The result is a composite amplifier with most

of the benefits of a differential amplifier without the loss in

system bandwidth.

Figure 1

Figure 2

The positive and negative high voltage supplies on the MSK

611 can be adjusted to reduce power dissipation. The output

of the MSK 611 will typically swing to within 8V of either

output voltage power supply rail. Therefore, if the system in

question only needs the output of the amplifier to swing 0 to

40V peak, the power supply rails could be set to -15V and

+50V safely. For best performance, the minimum value of

nected to ground if the output does not need to swing through

zero volts. The high voltage and low voltage power supplies

should be decoupled as shown in Figure 1.

HIGH VOLTAGE SUPPLIES

Since the MSK 611 is a high voltage amplifier, it is com-

monly used in circuits employing large gains. Therefore, the

internal compensation was chosen for gains of -5V/V or greater.

In circuits running at gains of less than -5V/V, the user can

further compensate the device by adding compensation net-

works at the input or feedback node. Pin 1 (comp) should be

cations.

INTERNAL COMPENSATION

Transition time optimization of the MSK 611 follows the same

basic rules as most any other amplifier. Best transition times

will be realized with minumum load capacitance, minimum ex-

ternal feedback resistance and lowest circuit gain. Transition

times will degrade if the output is driven too close to either

supply rail. Feedback and input resistor values will affect tran-

sition time as well. See Figure 1 and Table 1 for recommended

component values.

TRANSITION TIMES

Rev. B 8/00

3