Hartley Oscillator Design with Enhancement MOSFET

 Oscillator are fundamental building block in RF circuits. So if you are RF circuit designer this could be useful to you. I have not tested the circuit yet but it should work. At least computer simulation proves this. Oscillator are not usually designed with MOSFET or I have not encountered many. Here the FET transistor used is the 2N7000 transistor. This is a enhancement type MOSFET. Such transistor requires positive gate to source voltage to operate. More on this was explained in the earlier tutorial Comparison of BJT, JFET, D-MOSFET, E-MOSFET construction and operation wherein construction difference and operation difference between types of transistor was briefly explained. 

Circuit Diagram & Animation

The following shows the animation of how the Hartley oscillator build with enhancement MOSFET 2N7000.works. 

enhancement mosfet hartley oscillator animation


Basically, an oscillator has a gain stage and a feedback stage. The gain stage is provided by the MOSFET. It should be properly biased. Here the 2N7000 MOSFET is biased in the active region. To bias the FET we have used voltage divider biasing method. MOSFET voltage divider biasing is very similar to BJT using voltage divider biasing. In the above circuit diagram, the resistor R1 and R2 forms the voltage divider circuit that provides the gate voltage. How to bias and calculate the value of these resistors R1, R2 and the drain resistor RD and source resistor RS is shown in Enhancement MOSFET Voltage Divider Biasing tutorial. That tutorial also shows how to calculate the coupling capacitors CC1 and CC2. A variable resistor VR of 1KOhm was used to vary the drain resistor to kick start the oscillator. Increasing the drain resistor value will increase the voltage gain of the MOSFET amplifier. This is evident from the voltage gain formula of the MOSFET biased using voltage divider biasing method. This is illustrated in the video demonstration below.

The feedback stage is the LC oscillator in case of Hartley and Colpitts oscillator. In basic Harley oscillator two inductors and one capacitor are used and the midpoint of these two inductors L1 and L2 in the above circuit are connected to the source. The resonance frequency of the LC tank circuit is the frequency of oscillation of the oscillator. The frequency or the L and C component values can be calculated using LC Parallel Resonant Circuit Online Calculator or the online oscillator calculator. With L1=L2=1mH and C=0.1uF the resonance frequency is 11.25 KHz.

The following shows the circuit diagram of Hartley Oscillator Design using Enhancement MOSFET.

enhancement mosfet hartley oscillator circuit diagram
The following shows the output signal from the oscillator on oscilloscope.

signal from enhancement mosfet hartley oscillator

Video Demonstration

The following demonstrates how the Hartley oscillator works with enhancement MOSFET.

In the video, we can see that when we increase the drain resistor value the amplitude of the oscillator signal output is increased. 

Further Recommendation

- Colpitts Oscillator Simulation in Proteus Tutorial

- How to Build BJT amplifier and test with Soundcard based PC Oscilloscope

- How to use Matlab Simulink as Oscilloscope 

- AM modulator using JFET transistor

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