Some question related to varactor diode bias circuit
Please see the word file that I've attached,
I'm currently designing a dielectric resonator oscillator operating at 7 GHz, which can be tuned by a varactor diode using a tuning line, which is separated from the main oscillator circuit.
I'm just wondering whether I can directly apply a bias voltage to the varactor through the bias tees (High impedance, 1/4 lambda lines with radial stub) without the use of a resistor in practical without the risks of damaging the diode? Since the varactor is configured in reverse-biased state and is on a separated circuit from the main oscillator circuit, the leakage current that flows through the diode should be very small. Do correct me if my concept is wrong
I'm sorry if the question sounds stupid or very basic. Your help is much appreciated. Thank you.
You don't need a resistor for biasing the varactor in your case. A resistor definitely helps, together with a decoupling cap, to lower the biasing noise entering into the oscillator circuit.
Technically....you do not need a series resistor. But are you sure it can never be reverse biased? You would have to be operating single supply to be 100% sure. If you have a dual supply op amp driving it....and one of the supplies was turned on before the other, you do not know what the drive voltage is. I pretty much always put a series 220 ohm in, unless I am 100% sure!
Hi,
Thank you for the helpful replies..
As for biff44 concerns, I think I'm going to use a single operating DC supply to bias the varactor diode..
Basically, the varactor diode, which I am using for the design is BB535 from Infineon, with a maximum reverse voltage of 30V (as stated in datasheet)
The reason I'm asking about the resistor is because I found out that when I'm simulating the varactor for its S11 (with the resistor attached), the tuning range seems to be locked (very small S11 phase change) when I varied the voltage from 0 to 28V. I can still get an oscillation around 7 GHz in the complete final circuit..but no tuning range can be obtained.
For the case with no resistor attached, there is a large S11 phase change when I varied the varactor bias voltage from 0 to 28V, and the final simulation gives an oscillation around 7 GHz with a overall tuning range of 23 MHz for 0-28V varactor bias
So the resistor question comes from the concern of varactor's current limiting and tuning capability issues