Electrical – I would like to design an op Amp based 25Mhz Hartley oscillator


Hello i am a beginner at this and i would like your guidance . I would like to design an op Amp based 25Mhz Hartley's oscillator . I did some calculations and came up with the oscillator below , but it doesn't oscillate please help meenter image description here

Best Answer

Andy aka has explained why the 741 type opamp is not a proper choice. You need an other opamp which has a unity gain bandwidth which is at least 10 times larger than your oscillation frequency. More than that, it must exhibit a very fast slew rate (large-signal bandwidth). This is not easy to find. For this reason, at this frequency a simple transistor stage may be a better choice.

However, besides these arguments: Even with a high-frequency opamp the shown circuit is not able to produce self-sustained oscillation. Look at the inductor L2 - what is its effect? It has (practically) no effect on the feedback network because it works just as a load impedance - nothing else.

Surprisingly, this error can be found in the literature rather often. The authors simply transfer the principle of transistor-based circuits to opamp based circuits - and they forget that the transistor acts as a current source, whereas the opamp is a voltage source.

Hence, it is important to add another resistor Ro between the opamp output and the common node of L2 and C1. In this case, we have a 3rd-order feedback path which allows the required 180 deg phase shift at the desired frequency (high pass Ro-L2-C1-L1)