6

FN1050.6

March 4, 2005

Application Information

Circuit Description

The schematic diagram details one amplifier section of the

CA3240. It consists of a differential amplifier stage using PMOS

source. This assures a high common-mode rejection ratio. The

output of the differential amplifier is coupled to the base of gain

supplies the required differential-to-single-ended conversion.

emitter-follower output stage. Pulldown for the output stage is

provided by two independent circuits: (1) constant-current-

voltage between the output terminal and V+. The dynamic

current sink becomes active whenever the output terminal is

more negative than V+ by about 15V. When this condition

sink current from the output terminal to V-. This current always

flows when the output is in the linear region, either from the

present. The purpose of this dynamic sink is to permit the

ground. When the load is returned to V+, it may be necessary

placing a resistor (Approx. 2k

Ω) between the output and V-.

Output Circuit Considerations

Figure 23 shows output current-sinking capabilities of the

CA3240 at various supply voltages. Output voltage swing to

the negative supply rail permits this device to operate both

power transistors and thyristors directly without the need for

level-shifting circuitry usually associated with the 741 series

of operational amplifiers.

Figure 3 shows some typical configurations. Note that a series

resistor, RL, is used in both cases to limit the drive available to

the driven device. Moreover, it is recommended that a series

diode and shunt diode be used at the thyristor input to prevent

large negative transient surges that can appear at the gate of

thyristors, from damaging the integrated circuit.

Input Circuit Considerations

As indicated by the typical VICR, this device will accept

inputs as low as 0.5V below V-. However, a series current-

limiting resistor is recommended to limit the maximum input

terminal current to less than 1mA to prevent damage to the

input protection circuitry.

Moreover, some current-limiting resistance should be

provided between the inverting input and the output when

the CA3240 is used as a unity-gain voltage follower. This

resistance prevents the possibility of extremely large input-

signal transients from forcing a signal through the input-

protection network and directly driving the internal constant-

current source which could result in positive feedback via the

output terminal. A 3.9k

Ω resistor is sufficient.

The typical input current is on the order of 10pA when the

inputs are centered at nominal device dissipation. As the

output supplies load current, device dissipation will increase,

raising the chip temperature and resulting in increased input

current. Figure 4 shows typical input-terminal current versus

ambient temperature for the CA3240.

CA3240

LOAD

30V NO LOAD

CA3240

LOAD

V+

+HV

CURRENT CAPABILITY OF THE CA3240 SERIES

CA3240, CA3240A