MSK610_15 Datasheet, PDF (3/4 Pages) M.S. Kennedy Corporation – Ultra Low Quiescent Current - ±12mA for High Voltage Stage

APPLICATION NOTES
FEED FORWARD TOPOLOGY
The MSK 610 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.
INTERNAL COMPENSATION
Since the MSK 610 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
bypassed with a 0.1uF ceramic capacitor to +VHV for all appli-
cations.
HIGH VOLTAGE SUPPLIES
The positive and negative high voltage supplies on the MSK
610 can be adjusted to reduce power dissipation. The output
of the MSK 610 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
+VHV should be +50VDC. The -VSC pin may be directly con-
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.
TRANSITION TIMES
Transition time optimization of the MSK 610 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.
VOLTAGE
GAIN
-10V/V
-20V/V
-50V/V
-RIN
RF
604Ω
301Ω
100Ω
6.04KΩ
6.04KΩ
5KΩ
Table 1
CF
0.5-2pF
N/A
N/A
CURRENT LIMIT
Figure 2 is an active short circuit protection scheme for the
MSK 610. The following formula may be used for setting cur-
rent limit:
Current Limit ≈ 0.6V / Rsc
RBASE must be selected based on the value of +VHV and -VCC
as follows:
RBASE = ((+VHV - (-VCC)) - 1.2V) / 4mA
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-
tion of CSC. The corresponding time constant would be:
T = (RSC) (CSC)
A common value for CSC is approximately 1000pF. If current
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.
Figure 1
Figure 2
3
Rev. A 8/00