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?

 

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

 

Ad1 CB amplifier input resistance (rin = 1/re ≈ 1/gm) do not load the tank circuit directly. Notice that rin is connected parallel with one of a feedback capacitors. Also LC tank circuit "see" rin as a larger resistance thanks to "capacitive transformer action".
http://www.inictel-uni.edu.pe/sites...pplications_of_the_capacitive_transformer.pdf
http://ocw.mit.edu/courses/electric...-circuits-spring-2005/lecture-notes/lec14.pdf (page 27)

 

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

 

With respect to b) maybe this will clear things up ... https://en.wikipedia.org/wiki/Choke_(electronics)

Read https://en.wikipedia.org/wiki/Colpitts_oscillator and http://www.electronics-tutorials.ws/oscillator/colpitts.html

 

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?

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?

b)what is the function of rf choke in fig 22-13?

Isolation of RF from DC power.

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"".

Why do you think the tank is not loaded?

 

why the text says that c1 and c2 are in series if they are grounded in between?

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

b)what is the function of rf choke in fig 22-13?

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).

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

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

 

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

 

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.

 

Brownout , I drew what I understood, please refer to picture attached (tank png) Can someone correct me please?

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.

 

"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

 

" 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

curret flows back and forth through the tank circuit.

 

ok!
thanks
sm

 

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

In parallel LC circuit we have "one" circulating current between the inductor and the capacitor.

The charged capacitor is now connected in parallel across the inductive coil so the capacitor begins to discharge itself through the coil. The voltage across C starts falling as the current through the coil begins to rise. This rising current sets up an electromagnetic field around the coil which resists this flow of current. When the capacitor, C is completely discharged the energy that was originally stored in the capacitor, C as an electrostatic field is now stored in the inductive coil, L as an electromagnetic field around the coils windings.

As there is now no external voltage in the circuit to maintain the current within the coil, it starts to fall as the electromagnetic field begins to collapse. A back emf is induced in the coil (e = -Ldi/dt) keeping the current flowing in the original direction.

This current charges up capacitor, C with the opposite polarity to its original charge. C continues to charge up until the current reduces to zero and the electromagnetic field of the coil has collapsed completely.

The energy originally introduced into the circuit through the switch, has been returned to the capacitor which again has an electrostatic voltage potential across it, although it is now of the opposite polarity. The capacitor now starts to discharge again back through the coil and the whole process is repeated. The polarity of the voltage changes as the energy is passed back and forth between the capacitor and inductor producing an AC type sinusoidal voltage and current waveform.

https://en.wikibooks.org/wiki/Circuit_Idea/How_do_We_Create_Sinusoidal_Oscillations?#Operation

 

Dear Jony130 and all members

Thanks a lot!