LMH6683 Datasheet, PDF (16/30 Pages) Texas Instruments – 190MHz Single Supply, Dual and Triple Operational Amplifiers

LMH6682, LMH6683
SNOSA43A – MAY 2004 – REVISED APRIL 2013
APPLICATIONS SECTION
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LARGE SIGNAL BEHAVIOR
Amplifying high frequency signals with large amplitudes (as in video applications) has some special aspects to
look after. The bandwidth of the Op Amp for large amplitudes is less than the small signal bandwidth because of
slew rate limitations. While amplifying pulse shaped signals the slew rate properties of the OpAmp become more
important at higher amplitude ranges. Due to the internal structure of an Op Amp the output can only change with
a limited voltage difference per time unit (dV/dt). This can be explained as follows: To keep it simple, assume
that an Op Amp consists of two parts; the input stage and the output stage. In order to stabilize the Op Amp, the
output stage has a compensation capacitor in its feedback path. This Miller C integrates the current from the
input stage and determines the pulse response of the Op Amp. The input stage must charge/discharge the
feedback capacitor, as can be seen in Figure 51.
Figure 51.
When a voltage transient is applied to the non inverting input of the Op Amp, the current from the input stage will
charge the capacitor and the output voltage will slope up. The overall feedback will subtract the gradually
increasing output voltage from the input voltage. The decreasing differential input voltage is converted into a
current by the input stage (Gm).
I*Δt = C *ΔV
(1)
ΔV/Δt = I/C
(2)
I=ΔV*Gm
(3)
where I = current
t = time
C = capacitance
V = voltage
Gm = transconductance
Slew rate ΔV/Δt = volt/second
In most amplifier designs the current I is limited for high differential voltages (Gm becomes zero). The slew rate
will than be limited as well:
ΔV/Δt = Imax/C
(4)
The LMH6682/83 has a different setup of the input stage. It has the property to deliver more current to the output
stage when the input voltage is higher (class AB input). The current into the Miller capacitor exhibits an
exponential character, while this current in other Op Amp designs reaches a saturation level at high input levels:
(see Figure 52)
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