晶闸管电源
Amplifiers: Op Amps
Texas Instruments Incorporated
Although all of these ferrite chips have
the same impedance at 100 MHz (600 ),they produced different results. The HDseries high-Q chip shows a very narrow
and large peak that will most likely result
in instability and oscillations. The AG andHG series low-Q chips both performed
about the same, and either one would
probably produce acceptable results. The
only difference is that the HG series has
impedance at higher frequencies andwould probably be better suited for usewith very high-speed CFB amplifiers suchas the OPA685 or the THS3202.
Notice that the pure resistance has a
lower response peak than the ferrite chips.
Coupled with the fact that the HD series
has a high Q and a high peak, this impliesthat the slope of the impedance at the
amplifier’s bandwidth is a factor for stabil-ity. This makes a lot of sense; as it is well
known that for any amplifier, if a zerointersects the amplifier’s open-loopresponse at a rate of closure of 40 dB/decade, large peaking and oscillations willmost likely result.5For this circuit config-uration, if the impedance of Z has a largeslope that intersects the transimpedancecurve at essentially a rate of closure of 40 dB/decade, peaking and oscillationsalso will most likely occur. By comparison,a resistor intersects the transimpedancecurve at a rate of closure of 20 dB/decade,resulting in a stable response. Even though
the low-Q ferrite beads have some slope
related to their impedance, the rate ofclosure is much lower than 40 dB/decade,
providing improved stability. Nevertheless,
minimizing this intersection rate of closureas much as possible should produceacceptable results.To further expand on the usefulness of
the ferrite chips, more testing was doneutilizing the AG series in the circuit,asshown in Figure 6.
This figure shows that, just like the
results for the pure resistor, the higher
the impedance is, the lower the peaking.
How does this affect the output noise ofthe system? Figure 7 shows the outputnoise when the ferrite chips were used,along with the output noise of the THS4012
and some of the original resistor configurations.Inverting gain configurationAs expected, due to the low frequency impedance of theAll of the testing discussed so far was done with the non-ferrite chips, the noise is extremely low. This noise was theinverting gain configuration. This configuration forces thesame regardless of which ferrite was used. If noise aboveinverting node voltage to move proportionally to the input10 MHz was important, the impedance of these ferritevoltage applied. So how does the system work in thechips would start to increase the output noise to the sameinverting gain configuration where the inverting node isextent as resistors. These tests show that there are severalheld at a virtual ground? The easy answer is that it worksadvantages of using ferrite chips over resistors.