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扩展电流反馈放大器的可用性(4)

发布时间:2021-06-05   来源:未知    
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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.

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