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Design IdeasJanuary 2, 1997 |
Certain electronic circuits require extremely
low-noise power supplies. Low-phase-noise phase-locked loops and high-gain
preamplifiers are two examples. "Low noise" here implies
self-generated noise as well as unregulated-dc-line noise. The regulator in Figure 1 exhibits low noise in both these
regards, yet is reasonably simple, considering its features. The regulator uses
an op amp to amplify a stable voltage reference, VREF. With control
transistor Q1 selected for adequate output current, the circuit
produces a regulated output, VOUT, determined by the following
expression:
where VREF is the reference voltage of D1.
Although this basic approach of bootstrapped-reference regulator design is more than 20 years old, some wrinkles in this circuit set it apart. First, the use of a current source to drive the pass device Q1 yields relatively low dropout (approximately 1.5V), plus the ability to bootstrap the op-amp power from the regulated output. The latter trait is particularly helpful, because it affords greater than 100-dB supply rejection in the audio range, or well above frequencies where most op amps suffer from declining supply rejection. As with classic forms of this regulator, the reference diode D1 derives its feed from the output, thereby eliminating supply-line dependence. Heavy noise filtering of VREF by R1 and C4 reduces the diode's typical 100 nV/÷Hz to the equivalent of a few nV/÷Hz (at 1 kHz) at IC1's output.
To achieve low noise, amplifier IC1
must have low input-voltage noise density; in this case, 1 nV/÷Hz for the
AD797. But the topology itself should also minimize ac gain to hold down noise
increases. C3 reduces the ac gain of IC1 to unity, with
dc gain set by the R4, R3 values. The result is that the
overall regulator circuit can operate with output noise on the order of about 3
nV/÷Hz
at 1 kHz. Line-rejection performance is somewhat difficult to assess. In a
specialized high-resolution setup, the regulator's rejection measures within a
couple of decibels of test-equipment residual noise, as shown in Figure 2.
In comparison with the same IC1 device operated without supply bootstrapping (Reference 1), the relative noise improvement with bootstrapping is 30 to 35 dB and is frequency-dependent. The bootstrapped performance is high enough to be comparable with the use of a preregulator, but without its complexity. The output impedance is approximately 1 m[ohms] at 100 kHz, dropping to a few microhms below 100 Hz. Despite the obvious performance virtues, bootstrapping the reference and op amp is not without some serious application caveats—for example, providing a positive guarantee of circuit startup.
First, you must use the op amp in a single-supply mode as shown to prevent possible output state reversal. Second, you should choose level-shift zener diode D3 for a certain fall-back criterion; namely, the voltage that the output would fall back to should the op amp not initially bias properly. The criterion should provide that, even if IC1 should momentarily come up in a low-output state, the net bias of the Q3-D3 string will still be greater than VREF. Then, with IC1 not yet fully active, the bias voltage at Q1 will force VOUT to start positive. Once VREF is exceeded, IC1 gains control, and the circuit achieves its desired stable state. A suitable selection criterion for D3 is to simply make its breakdown voltage similar to that of D1, in this case approximately equal to 6.8V.
As shown, the circuit can supply approximately 250 mA with a nominal output of twice VREF, or approximately 14V. If you need remote sensing, you can add the remote-sense isolation resistor R2 at the load point. Breaking the normal sense line at X enables the remote-sensing option, with C1 added to decouple the sense loop at high frequencies. A negative-output version reverses the diodes and capacitors, along with the op-amp supply pins, and substitutes complementary transistors. (DI #1974)
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