# Common base Colpitts oscillator

Discussion in 'Homework Help' started by Saviour Muscat, Jun 23, 2015.

1. ### Saviour Muscat Thread Starter Member

Sep 19, 2014
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Hello members and guests
I am studying a common base Colpitts oscillator, I encounter some difficulties, please someone give a hand
a) Why the tank circuit is not loaded at high frequency(common base have low input impedance)?
b)In this type of circuit what is the use of RF choke?

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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In every type of circuit, a RF Choke has the same purpose. Do you see where the RF Choke is in your circuit?

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3. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Last edited: Jun 24, 2015
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4. ### Saviour Muscat Thread Starter Member

Sep 19, 2014
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a)please refer to the figure attached fig 22-11b, why the text says that c1 and c2 are in series if they are grounded in between?
b)what is the function of rf choke in fig 22-13?
c)again refer to fig 22-11b, in the previous intervention, you said in words that the amplifier is not loaded because at the input R3 is parallel with C2 ,Am I correct? and I didn't understood well the quoted "Also LC tank circuit "see" rin as a larger resistance thanks to "capacitive transformer action"".
I tried to study the links but they little hard for me learn them
thanks
sm

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Apr 26, 2005
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6. ### Brownout Well-Known Member

Jan 10, 2012
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Draw another ground just below the one shown in the diagram. Now, break the line between the two grounds. Since that line only connected two grounds, breaking it won't change the circuit. Now, redraw the tank with C1 on the left side, connected through L1 to C2 on the right side. Now can you see why they are in series?

Isolation of RF from DC power.

Why do you think the tank is not loaded?

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7. ### Jony130 AAC Fanatic!

Feb 17, 2009
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But if we looking from the inductor coil terminal (point of view) the C1 and C2 are connected in series.

RF choke is nothing more than a ordinary inductor that has a high inductance value. This means that the inductance reactance XL will also have a high value. So this implies that RFC will pass DC current needed to supply transistor and block any AC signal. Thanks to this a high frequency output signal are blocked from reaching the power supply (chock has a very high inductance reactance).

Rin will load the tank circuit, but this loading do not have much effect on tank circuit.

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8. ### Saviour Muscat Thread Starter Member

Sep 19, 2014
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Thanks to all, especially to Jony130

"Draw another ground just below the one shown in the diagram. Now, break the line between the two grounds. Since that line only connected two grounds, breaking it won't change the circuit. Now, redraw the tank with C1 on the left side, connected through L1 to C2 on the right side. Now can you see why they are in series?"
Regarding the post left by Brownout , I drew what I understood, please refer to picture attached (tank png) Can someone correct me please?

"""c)again refer to fig 22-11b, in the previous intervention, you said in words that the amplifier is not loaded because at the input R3 is parallel with C2 ,Am I correct? and I didn't understood well the quoted "Also LC tank circuit "see" rin as a larger resistance thanks to "capacitive transformer action"".
I made a mistake the right figure is fig22-13, "Rin will load the tank circuit, but this loading do not have much effect on tank circuit" Is this statement applies for fig22-13 common base? Kindly state yes or no, if no can you please explain!

Many thanks to all your help

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9. ### Jony130 AAC Fanatic!

Feb 17, 2009
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R3 is not Rin for CB amplifier. Rin for Cb amp is Rin = 1/re||R3 ≈ 1/re ≈ 26mV/Ie.
And YES, this statement applies for fig22-13 too.

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10. ### Brownout Well-Known Member

Jan 10, 2012
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That's it. Remember that current in a tank flows back and forth through the tank. So from that prespective, C1 and C2 are in series.

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11. ### Saviour Muscat Thread Starter Member

Sep 19, 2014
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"R3 is not Rin for CB amplifier. Rin for Cb amp is Rin = 1/re||R3 ≈ 1/re ≈ 26mV/Ie."
I think re<<R3 to satisfied this equation

" tank flows back and forth through the tank"
could you describe the current flow (refer to components and pionts in the image attached)

Grateful thanks

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12. ### Brownout Well-Known Member

Jan 10, 2012
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curret flows back and forth through the tank circuit.

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Sep 19, 2014
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ok!
thanks
sm

14. ### Jony130 AAC Fanatic!

Feb 17, 2009
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In parallel LC circuit we have "one" circulating current between the inductor and the capacitor.
https://en.wikibooks.org/wiki/Circuit_Idea/How_do_We_Create_Sinusoidal_Oscillations?#Operation

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15. ### Saviour Muscat Thread Starter Member

Sep 19, 2014
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Dear Jony130 and all members

Thanks a lot!