Micrel, Inc.

MIC4721

May 2007

13

M9999-052907-A

The network analyzer will then sweep the source while

monitoring A and R for an A/R measurement. While this

is the most common method for measuring the gain and

phase of a power supply, it does have significant

limitations. First, to measure low frequency gain and

phase, the transformer needs to be high in inductance.

This makes frequencies <100Hz require an extremely

large and expensive transformer. Conversely, it must be

able to inject high frequencies. Transformers with these

wide frequency ranges generally need to be custom

made and are extremely expensive (usually to the tune

of several hundred dollars!). By using an op-amp, cost

and

frequency

limitations

caused

by

an

injection

transformer

are

completely

eliminated.

Figure

8

demonstrates using an op-amp in a summing amplifier

configuration for signal injection.

Figure 8. Op Amp Injection

R1 and R2 reduce the DC voltage from the output to the

non-inverting input by half. The network analyzer is

generally a 50

Ω source. R1 and R2 also divide the AC

signal sourced by the network analyzer by half. These

two signals are “summed” together at half of their

original input. The output is then amplified by 2 by R3

and R4 (the 50

Ω is to balance the network analyzer’s

source impedance) and sent to the feedback signal. This

essentially breaks the loop and injects the AC signal on

top of the DC output voltage and sends it to the

feedback. By monitoring the feedback “R” and output

“A”, gain and phase are measured. This method has no

minimum frequency. Ensure that the bandwidth of the

op-amp being used is much greater than the expected

bandwidth of the power supply’s control loop. An op-amp

with >100MHz bandwidth is more than sufficient for most

power

supplies

(which

includes

both

linear

and

switching) and are more common and significantly

cheaper than the injection transformers previously

mentioned. The one disadvantage to using the op-amp

injection method; is the supply voltages need to be

below the maximum operating voltage of the op-amp.

Also, the maximum output voltage for driving 50

Ω inputs

using the MIC922 is 3V. For measuring higher output

voltages, a 1M input impedance is required for the A and

R channels. Remember to always measure the output

voltage with an oscilloscope to ensure the measurement

is working properly. You should see a single sweeping

sinusoidal waveform without distortion on the output. If

there is distortion of the sinusoid, reduce the amplitude

of the source signal. You could be overdriving the

feedback causing a large signal response.

The following Bode analysis show the small signal loop

stability of the MIC4721. The MIC4721 utilizes a type III

compensation. This is a dominant low frequency pole,

followed by 2 zero’s and finally the double pole of the

inductor capacitor filter, creating a final 20dB/decade roll

off. Bode analysis gives us a few important data points;

speed of response (Gain Bandwidth or GBW) and loop

stability. Loop speed or GBW determines the response

time to a load transient. Faster response times yield

smaller voltage deviations to load steps. Instability in a

control loop occurs when there is gain and positive

feedback. Phase margin is the measure of how stable

the given system is. It is measured by determining how

far the phase is from crossing zero when the gain is

equal to 1 (0dB).

•

Phase Margin = 47 Degrees

•

GBW = 156KHz

Gain will also increase with input voltage. The following

graph shows the increase in GBW for an increase in

supply voltage.