P r o d u c t I n n o v a t i o n F r o m

MP400

10

MP400U

Current can be injected into the output stage by adding a resistor, R

depend upon the application but 500 µA (50 V V+ supply/100K) of added bias current shows significant improve-

ment in the small signal phase plots. Adding this resistor has little to no impact on small signal gain or large signal

performance as under these conditions the current in the input stage is elevated over its idle value. It should also

be noted that connecting a resistor to the upper supply only injects a fixed current and if the upper supply is fixed

and well bypassed. If the application includes variable or adjustable supplies, a current source diode could also be

used. These two terminal components combine a JFET and resistor connected within the package to behave like

a current source.

As a second stability measure, the PA78 is externally compensated and performance can be optimized to the ap-

plication. Unlike the R

the large signal response of the amplifier. Refer to the small and large signal response plots as a guide in making

the tradeoffs between bandwidth and stability. Due to the unique design of the PA78, two symmetric compensation

networks are required. The compensation capacitor C

supply voltage (+V

temperature.

The PA78 requires an external 33 pF capacitor between C

output. This capacitor should be rated for the full supply voltage (+V

larGe siGnal PerForMance

As the amplitude of the input signal increases, the internal dynamic current sources increase the operation band-

width of the amplifier. This unique performance is apparent in its slew rate, pulse response, and large signal perfor-

mance plots. Recall the previous discussion about the relationships between signal amplitude, supply current, and

slew rate. As the amplitude of the input amplitude increases from 1 V

V/µs to well over 350 V/µs.

Notice the knee in the Rise and Fall times plot, at approximately 6 V

becomes clipped by the supply rails and the amplifier is no longer operating in a closed loop fashion. The rise and

fall times become faster as the dynamic current sources are providing maximum current for slewing. The result of

this amplifier architecture is that it slews fast, but allows good control of overshoot for large input signals. This can

be seen clearly in the large signal Transient Response plots.

heatsinkinG anD saFe oPeratinG area

The MOSFET output stage of the PA78 is not limited by second break-

down considerations as in bipolar output stages. Only thermal con-

siderations of the package and current handling capabilities limit the

Safe Operating Area. The SOA plots include power dissipation limita-

tions which are dependent upon case temperature. Keep in mind that

the dynamic current sources which drive high slew rates can increase

the operating temperature of the amplifier during periods of repeated

slewing. The plot of supply current vs. input signal amplitude for a 100

kHz signal provides an indication of the supply current with repeated

slewing conditions. This application dependent condition must be con-

sidered carefully.

The output stage is self-protected against transient flyback by the para-

sitic body diodes of the output stage. However, for protection against

sustained high energy flyback, external, fast recovery diodes must be

used.

cUrrent liMit

For proper operation, the current limit resistor, R

gram. For maximum reliability and protection, the largest resistor value should be used. The maximum practical

value for R

timum value for Rlim in the intended application. Current limit may not protect against short circuit conditions with

supply voltages over 200 V.

SOA

0

20

40

60

80

100

120

140

160

10

100

1000

SUPPLY TO OUTPUT DIFFERENTIAL, V

25°C

125°C

75°C