AN1103: Using Single-Supply High-Frequency Amplifiers

Using Single-Supply High-Frequency Amplifiers
®
Application Note
March 23, 1998
AN1103
Author: Barry Harvey and Mike Wong
The trend toward lower power supply
voltages has recently inspired the
design of integrated single-supply
high-frequency amplifiers. The greatest use of these circuits
is in passing video signals, although there are many other
applications. Unity-gain bandwidths of the new offerings are
in the 90MHz–160MHz range.
The earliest single-supply IC amplifier is the LM324,
developed circa 1974. This is a 1MHz op-amp that set the
definition of a single-supply circuit:
A single-supply device works with no minus supply; using
0V and typically +5V for minus and V+ supply voltages,
respectively. Inputs will linearly comply with 0 (ground) to
some voltage approaching V+. Outputs will provide from
0V or nearly so to a voltage approaching V+.
Figure 1 shows circuitry at the input of these amplifiers. The
input PNP differential pair is followed by an NPN folded
cascode that performs level-shifting. The cascode emitter
resistors have typically only 200mV across them, allowing
the input PNP bases to comply with ground-level inputs, and
even a bit below ground, without severe saturation. For
positive swings, the current-source transistors and the VBE's
of the input transistors set the input positive headroom,
between 1.1V and 1.7V. The DC input offset will not degrade
until input swings are within millivolts of the headroom limits,
but AC characteristics can vary hundreds of millivolts from
the limits. Usually bandwidth is the non-constant parameter,
but in some circuits transient responses are also distorted.
FIGURE 1. TYPICAL SINGLE-SUPPLY
AMPLIFIER INPUT STAGE
The maximum output swing below V+ usually is about the
same as input headroom. A complication is that output
stages in bipolar technology cannot inherently sink current
all the way to ground, and minimum output low swing
depends on the polarity and magnitude of load current. A
grounded pull-down resistor is often used to help the output
to drop all the way to ground. Figure 2A shows an NPN/PNP
output stage. A load resistor keeps the output NPN properly
biased all the way to ground, but the output PNP has turned
off well above. Figure 2B shows an all-NPN output stage.
This design cannot swing as close to ground as 2A with a
pull-down resistor, but it can swing somewhat lower than 2A
when sinking load current. The all-NPN circuit does exhibit
generally greater distortions and load capacitance sensitivity
than the NPN-PNP design.
FIGURE 2. TYPICAL SINGLE-SUPPLY OUTPUT STAGES
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Application Note 1103
FIGURE 3. DIFFERENTIAL GAIN AND PHASE ERROR, AV = 1, RL = 500,
VIN = 0V–5V, DG = 0.2%/DIV, DP = 0.2°/DI
Output linearities are generally quite good within the
headroom range for DC signals, but high AC distortions
occur when levels approach clipping. Figure 3 shows the
differential gain and phase errors, that is, the constancy of
small-signal gain and phase as DC level varies, for several
commercial amplifiers. Amplifier A is an EL2044, a generalpurpose video amplifier; B is an EL2210, a low-cost video
amplifier; C is an EL2150, a 125MHz true single-supply
video amplifier; and amplifier D is another vendor's 90MHz
single-supply amplifier. In Figure 3, all amplifiers are wired
for a gain of +1, the input is a 50mVP-P 3.58MHz AC signal
with DC offset sweeps from 0V to 5V, the load is 500Ω to
ground, and the supply voltage is +5V. Here is a summary of
amplifier swings, with 0.4% or 0.4° gain and phase
deviations from the center of the span:
TABLE 1. AMPLIFIER OUTPUT SWINGS [V] FOR 0.4% OR
0.4° DEVIATION FROM CURVES IN FIGURE 3
AMPLIFIER
LOW
SWING
HIGH
SWING
SPAN
CENTER
TOTAL
SPAN
A
1.05
3.00
2.03
1.95
B
0.65
2.80
1.73
2.15
C
0.22
3.50
1.86
3.28
D
0.22
4.10
2.16
3.88
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For small signals, differential gain is similar to linearity error
or harmonic distortion, although the scale factor between
them is not 1:1. Differential phase is a specification important
to preserving color quality in NTSC video. Differential gain
and phase errors of 0.5% and 0.5° are sufficient for
consumer or overlay NTSC video; 0.1% and 0.1° or better is
required for professional NTSC video; and 0.5% gain
linearity is sufficient for high-quality component video
signals.
Note that each amplifier has a unique set of swings and
center of span. Note that the single-supply amplifiers simply
cannot operate linearly all the way to ground. If the
maximum linear spans are to be obtained, the input signal
must be offset by the Span Center. If offset, all of these
amplifiers can pass an NTSC video signal with low distortion,
even the non-single-supply models.
Figure 4 shows the amplifiers wired for a gain of +2 and
driving a 150Ω load to ground, as in driving a backterminated 75Ω cable service. The input is a 50mVP-P
3.58MHz AC signal with DC offset sweeps from 0V to 2.5V.
The amplifiers all have more distortion with the heavier load:
Application Note 1103
FIGURE 4. DIFFERENTIAL GAIN AND PHASE ERROR, AV = 2, RL = 150,
VIN = 0V–2.5V, DG = 0.1%/DIV, DP = 0.2°/DIV
TABLE 2. AMPLIFIER OUTPUT SWINGS [V] FOR 0.4% OR
0.4° DEVIATION FROM CURVES IN FIGURE 4
TABLE 3. AMPLIFIER OUTPUT SWINGS FOR 0.4% OR 0.4°
DEVIATION FROM CURVES IN FIGURE 5
AMPLIFIER
LOW SWING
HIGH
SWING
SPAN
CENTER
TOTAL
SPAN
AMPLIFIER
LOW
SWING
A
1.05
2.50
1.78
1.45
A
1.70
2.83
2.27
1.13
B
0.50
2.15
1.33
1.65
B
2.45
2.80
2.63
C
0.30
3.13
1.72
2.83
0.35 — requires
pull-down
D
0.45
4.00
2.23
3.55
C
1.15
3.45
2.30
2.30 — 0.5%/0.5°
D
0.73
2.00
1.36
1.27
The amplifier A cannot linearly swing the 2V span of doublesized NTSC signal and is not applicable to this task.
Amplifier B can be used if the input is offset such that the
worst distortion occurs in the non-critical sync portion of the
signal. Neither of the true single-supply amplifiers will have
trouble with video signals, and the output can be allowed to
go all the way to ground in the sync tip.
The worst trouble occurs when the load is capacitor-coupled.
This is very common for instrument inputs and outputs. In
this case, AC load currents can be positive or negative.
Figure 5 shows the performance of the amplifiers driving
300Ω to ground and 300Ω to V+, emulating a capacitorcoupled 150Ω load.
Generally, then, in single-supply amplifier circuits, signals
will be offset above ground with capacitor coupling and DCrestoration used to regain offset levels.
3
HIGH
SPAN
SWING CENTER
TOTAL SPAN
Again amplifier A does not have enough swing for our video
signal. Amplifier B fails altogether without a pull-down
resistor. With the pulldown B's performance will be very
close to that of Figure 4. Amplifier C shows crossover
distortion but has a good swing, if we allow the error to
increase to 0.5% and 0.5°. Amplifier D has terrible crossover
distortion and a very restricted output range, so is useless
for driving bi-directional load currents.
Application Note 1103
FIGURE 5. DIFFERENTIAL GAIN AND PHASE ERROR, AV = 2, RL = 300 TO VCC AND
300 TO GROUND, VIN = 0V–2.5V, DG = 0.1%/DIV, DP = 0.2°/DIV
The transient response curves shown in Figure 6 present
another approach to studying distortion when the output is
driven toward ground. All amplifiers are wired in unity gain
configuration and powered with a +5V single supply and
drive 150Ω load. The input is a 1MHz 4VP-P sinewave with
its minimum voltage at 0V. Amplifier
A output clips are around 250mV. Amplifiers B and C output
linearly go down to 50mV above ground. Amplifier D, even
though it can reach 80mV from ground, shows a saturation
recovery step at 475mV. Voltage clipping and saturation
recovery directly translate to differential gain and phase
errors.
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Application Note 1103
FIGURE 6. TRANSIENT RESPONSE, AV = 1, RL = 150
Conclusion
High-frequency op-amps' linearities suffer greatly in singlesupply service. To optimize video performance it is helpful to
level-shift the signal, possibly with a DC-restore system.
Specially designed single-supply amplifiers offer the widest
swings, but have serious limitations and behaviors that must
be measured individually.
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to
verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
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