I have a 30 MHz colpitts oscillator whose output I want to feed into a 50 ohm input impedance amplifier. When I try to simulate it the oscillator works fine by itself, however when I connect a 50 ohm ouput load the oscillation dies. My thinking was that the 50 ohm load was excessively loading the oscillator, so I first tried to add an emitter follower buffer but it didn't work. Then I tried a class B output stage but that didn't work either. Can somebody point me to how may I feed the oscillation to an amplifier with out loading it?enter image description here

  • \$\begingroup\$ Why so little collector current? R9 is far larger than needed. Since Vcc is unspecified, I hesitate to suggest a value, but perhaps 1k might work. If you want to feed oscillations to 50 ohm load, try connecting C8 to C7 instead - it is a low-impedance point. \$\endgroup\$
    – glen_geek
    Commented May 21 at 12:37
  • \$\begingroup\$ You should divide R7, R8, R9 C9 by a factor of ten ... This would allow you driving a load on "vout" of 500 Ohm. \$\endgroup\$
    – Antonio51
    Commented May 21 at 13:04
  • \$\begingroup\$ Try splitting L2 into a 500n + 50n (or some other big/small ratio), with the small one to the +ve rail, and take the resulting low impedance output from their junction. If glen's suggestion of C7 doesn't work, try splitting C7 into a big+small, say 1 nF series 10 nF, with the 10n to ground, and take the output wrt ground from their junction. \$\endgroup\$
    – Neil_UK
    Commented May 21 at 13:08
  • \$\begingroup\$ Something to remember. It is often harder to get an oscillator to work in simulation than in real life. \$\endgroup\$ Commented May 21 at 14:53

1 Answer 1


Neil's suggestion (in comments) is probably the best way to extract power from this oscillator. Have added a few more.

Be aware that delivering power to 50 ohms will be somewhat feeble, especially when biased with low transistor collector/emitter current. One has flexibility in choosing the emitter bias resistor to set delivered power. Q1 (below) is biased so that collector can swing down to nearly +2V.
Ideally one wishes to have collector voltage swing down close to base voltage at the peak of collector current without forward-biasing CB junction. This is a matter of balancing AC power delivered to the load resistor against oscillator power replenished by the transistor. One can match the 50 ohm load to a higher impedance seen at transistor collector in various ways:


simulate this circuit – Schematic created using CircuitLab

In the leftmost circuit, OP's C6 (52pf) has been divided between C1 (39pf) and C4 (22pf). C4 provides an impedance match to Rload so that collector is not loaded too much. About 5mW is delivered if Vcc is +9V. Be aware that C4 affects two circuit functions: it couples oscillator power out, and also affects frequency along with C1.

In the rightmost circuit, magnetic coupling is used to deliver power and to impedance-match to Rload of 50 ohms. Here, it is assumed near-perfect coupling close to one...in practice coupling is often poorer, so that L3 might be larger.
Neil has also suggested tapping L2 close to its Vcc connection point, forming an autotransformer. This is very similar to the magnetic coupling shown. Since output power is now driven from Vcc point, a coupling capacitor is required to deliver power to a ground-connected load. Power delivered is similar to left circuit.

When resonating collector inductor is loaded too much (below about 300 ohms) oscillation ceases. OP's emitter follower attempts to boost 50 ohm load to something higher. It is not a class-A amplifier - loading is asymmetric and seems to cause amplitude and frequency instability. If you intend to use a buffer stage, it should operate linearly (Class-A).

Do be careful to add a low-Z capacitor from Vcc to GND (not shown in these schematics).


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