1) It is single supply
,
No, I don't know that but I also don't know why it is so complicated. Is the opamp selection problematic? Or I need extra circutry to use it? I also don't know split supply concept.
Can you please give an example?
Thanks.
For symmetrical (dual) supply the designed operating point (opamp dc output) is approx. zero volts - thus your signal output can swing to both sides.
For single supply you must bias the opamp in order to have an operating point approx. at half of the supply voltage that allows again signal swing in both directions.
Remember the voltage divider at the base of a transistor in common emitter configuration.
For details you can find a lot of information via googling "single supply".
---------- Post added at 09:46 ---------- Previous post was at 09:44 ----------
But check if the whole circuit (PLL) works correctly with a quiescent filter dc output voltage.
Thanks LvW. I understand the configuration I attached from your words. If it is correct, I have already tried this configuration. I expect DC operating point of 5.2 V
and I bias the Vin+ at 3V. Unfortunately opamp pulls the output voltage to +Vcc.
View attachment 66087
Yeah - no surprise. This happens because you have no dc feedback. You need 100% dc feedback - only then the dc gain is unity and transfers the dc bias at the pos. input to the output.
Will it work if I connect a resistor from Vin+ to Vout?
Hello,
I want to design a PLL based oscillator. I am using ADF4157 as synthesizer and AD797A as active filter OPAMP.
Schematic of the structure is as given in the attachment.
I operate the opamp by supplying Vs+: 15V and Vs-: GND ; Vin+: GND
I am sure that ADF4157 is working correctly but I see 14V at the tuning port of oscillator in other words my active filter does not work. When I decrease Vs+, I always see 1V below Vs+ value at the tuning port (i.e. Vt=12V when Vs+=13V...). Situation did not change when I leave Vin+ opened or gave a reference voltage.
View attachment 66020
Do you have any idea about this problem?
Are you joking?
I suppose if you are working with PLL's you know some opamp basics.
You ask for a "suitable configuration"?
At first, you have to clarify
(a) what kind of loop filter you need (are there alternatives?)
(b) if you really have to live with single supply
(c) the input requirements of the VCO (nominal control voltage zero volts or something else?), because I suppose that capacitive coupling between loop filter and VCO does not work.
By the way this is a suggested filter in ADISim.
I did not know this. Actually I know what an opamp is but I am unfamiliar to usage of it. I was considering this structure as a standard thing that can be used easily. I am sorry about my very basic questions but do you have any suggestions about modification that can make this work?
I a not a specialist for PLL design - some other forum members certainly have more experience.
However, for my opinion the first question to be answered is (as mentioned before): What is the nominal VCO control voltage that must be provided by the loop filter?
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