Editing Clapp oscillator (section)

== Circuit ==
[[Image:Clapp oscillator.png|frame|Clapp oscillator (direct-current biasing network not shown)]]

The Clapp oscillator uses a single inductor and three capacitors to set its frequency. The Clapp oscillator is often drawn as a [[Colpitts oscillator]] that has an additional capacitor ({{math|''C''<sub>0</sub>}}) placed in series with the inductor.<ref>{{cite book |title=Basic Theory and Application of Transistors |date=1963 |orig-year=1959 |author=Department of the Army |pages=171–173|id=TM 11-690 |publisher=Dover |quote=Modification of the Colpitts oscillator by including a capacitor in series with winding 1–2 of the transformer results in the Clapp oscillator.}}</ref>

The oscillation frequency in Hertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor ([[Field effect transistor|FET]]), is
:<math>
 f_0 = {1 \over 2\pi}
       \sqrt{ {1 \over L}
              \left(   {1 \over C_0}
                     + {1 \over C_1}
                     + {1 \over C_2}
              \right)} \ .
</math>
The capacitors {{math|''C''<sub>1</sub>}} and {{math|''C''<sub>2</sub>}} are usually much larger than {{math|''C''<sub>0</sub>}}, so the {{math|1/''C''<sub>0</sub>}} term dominates the other capacitances, and the frequency is near the series resonance of {{math|''L''}} and {{math|''C''<sub>0</sub>}}. Clapp's paper gives an example where {{math|''C''<sub>1</sub>}} and {{math|''C''<sub>2</sub>}} are 40 times larger than {{math|''C''<sub>0</sub>}}; the change makes the Clapp circuit about 400 times more stable than the Colpitts oscillator for capacitance changes of {{math|''C''<sub>2</sub>}}.<ref>{{harvnb|Clapp|1948|p=357}}</ref>

Capacitors {{math|''C''<sub>0</sub>}}, {{math|''C''<sub>1</sub>}} and {{math|''C''<sub>2</sub>}} form a voltage divider that determines the amount of feedback voltage applied to the transistor input.

Although the Clapp circuit is used as a variable frequency oscillator ([[VFO]]) by making {{math|''C''<sub>0</sub>}} a variable capacitor, Vackář states that the Clapp oscillator "can only be used for operation on fixed frequencies or at the most over narrow bands (max. about 1:1.2)."<ref>{{harvnb|Vackář|1949|pp=5–6}}</ref> The problem is that under typical conditions, the Clapp oscillator's loop gain varies as {{math|''f''<sup>&nbsp;&nbsp;−3</sup>}}, so wide ranges will overdrive the amplifier. For VFOs, Vackář recommends other circuits. See [[Vackář oscillator]].

===Practical example===
[[File:Clapp Osz JFET Drain.gif|frame|Clapp oscillator practical example. Output frequency is 10&nbsp;MHz]]

The schematic shows an example with component values.<ref>{{cite book   
  | last = Hayward
  | first = Wes
  | title = Introduction to Radio Frequency Design
  | chapter = Figure 7.8 The Clapp variation of the Colpitts oscillator
  | publisher = ARRL
  | year = 1994
  | location = US
  | page = 274
  | url = https://archive.org/details/isbn_9780872594920
  | doi = 
  | id = 
  | isbn = 0-87259-492-0}}</ref> Instead of [[field-effect transistors]], other active components such as [[bipolar junction transistor]]s or [[vacuum tube]]s, capable of producing gain at the desired frequency, could be used.

The [[common-drain|common drain amplifier]] has a high input impedance and a low output impedance. Therefore the amplifier input, the gate, is connected to the high impedance top of the LC circuit C0, C1, C2, L1 and the amplifier output, the source, is connected to the low impedance tap of the LC circuit. The [[grid leak]] C3 and R1 sets the [[operating point]] automatically through [[Biasing#Grid_leak_bias|grid leak bias]]. A smaller value of C3 gives less [[Distortion#Harmonic_distortion|harmonic distortion]], but requires a larger load resistor. The supply current for J1 flows through the [[Choke_(electronics)#Radio_frequency_choke|radio frequency choke]] L2 to ground. The oscillator radio frequency current uses C2, because for the oscillator frequency this component has less [[Electrical reactance|reactance]]. The load resistor RL is part of the simulation, not part of the circuit.