Quick designing of AM transmitter, Understanding

[goldsmith]

In post#1. C3 is feedback capacitor?

Yes that is !
Eshal , now , i should go to bed and sleep , because this day was a heavy day for me and here in my city , now time is around 3:45 AM . hence good morning and see sweet dreams ! ( i will comeback and more help you )
Best Wishes
Goldsmith

 

[Eshal]

Thank you for today.
Good morning. take care

 

[goldsmith]

Hi Eshal
I came back again !
So , at last you understood what values for tank network and feed back capacitor , are required . now can we continue ?

 

[Eshal]

Yes, bro I have understood how to find value for feedback capacitor. Your method is very good. The technique i.e. Capacitor=0.5C of tank is very good.

Yes we can continue.

 

[goldsmith]

So , now something regarding biasing network and emitter impedance . as you probably know voltage gain for a CB amplifier will given by ZC/re . re will be 26mv/IC for 27 degree of room . so if IC will be increased it will be decreased (re ) thus gain will be increased . but you can't increase gain up to each value that you want ! because if you decrease RE , to reduce re , you will decrease feed back current ( through C fed ) because it's path up to ground is easier . so the reasonable idea is using a by pass capacitor with special value to reduce RE ( not completely ) thus AC collector current can be increased without higher DC current . but this capacitor should be limited too if you increase it , it will stop the oscillations because of limitation in fed signal ( bypassing it through ground will consequence , lower voltage fed into emitter of transistor )
So most reasonable value for that , is equal to the value of fed capacitor or a bit lower . and for emitter resistor , it can be something between 47 ohms up to 470 ohms . ( however we can calculate it , exactly , but it is a bit time consuming and hence each designer prefer to don't waste time) . hence we can randomly choose it between those values and see the result or using a potentiometer with that value to calibrate it . so for my suggestion a 180 ohms or a 220 ohms would be ok for you .
you understand my meaning ?

 

[Eshal]

So most reasonable value for that , is equal to the value of fed capacitor or a bit lower .

The capacitor uses to bypass RE. The value of this bypass capacitor may be equal or bit lower in value to the feedback capacitor. Is that are you trying to say?

it can be something between 47 ohms up to 470 ohms . ( however we can calculate it , exactly , but it is a bit time consuming and hence each designer prefer to don't waste time)

I know how to calculate value of RE. But if you are saying it is waste of time then my reply is positive too.

ence we can randomly choose it between those values and see the result or using a potentiometer with that value to calibrate it . so for my suggestion a 180 ohms or a 220 ohms would be ok for you .

You suggested 180ohms or 220ohms, these values are limited to this circuit which we are discussing here right now, or it is for every circuit in which RE is to be bypassed?

 

[goldsmith]

The capacitor uses to bypass RE. The value of this bypass capacitor may be equal or bit lower in value to the feedback capacitor. Is that are you trying to say?

Yes but it is not a complete AC bypass . it just will bypass a little of it !

I know how to calculate value of RE. But if you are saying it is waste of time then my reply is positive too.

No it's calculation in this case is completely different . it isn't like designing a simple class A stage .

You suggested 180ohms or 220ohms, these values are limited to this circuit which we are discussing here right now, or it is for every circuit in which RE is to be bypassed?

You can test this oscillator with these values for frequencies nearby 1GHZ , easily .

 

[Eshal]

Yes but it is not a complete AC bypass . it just will bypass a little of it !

Yes bro, I have understood this.

No it's calculation in this case is completely different . it isn't like designing a simple class A stage .

Thanks for clearification.

You can test this oscillator with these values for frequencies nearby 1GHZ , easily .

Are you referring to the oscillator which we designed tomorrow? If so, we designed that for 100KHz frequency and on the behalf we chose value of feedback capacitor, tank circuit.
If I test with 1GHz frequency then will I have to calculate all those values again?

 

[goldsmith]

If I test with 1GHz frequency then will I have to calculate all those values again?

No No No ! don't confuse ! i told you can increase it's frequency with these values of ZE . but i didn't tell that you are working at 1GHZ ! i told it is possible . but not for you ! you will need more experiences for those very very high frequencies .

I can remember that i told you 100 KHZ .
So , now lets go through DC bias resistors . are you ready ?

 

[Eshal]

Yes, I am ready

 

[goldsmith]

There are some ways , like calculations at class A area for base voltage divider . but again there is a simple way to prevent those calculations ! perhaps you won't believe but i can calculate an oscillator with each specifications that i need during some seconds !
A simple 4.7k resistor in series with a potentiometer would be ok ! you can calibrate it to have maximum amplitude . series resistor is to prevent damages . potentiometer can be between 200 k up to 1 MEG ohms .
But if you are in calculation way i can tell you it too . but the result wil be the same . if you want calculations you can remove one of the resistors , and just put a resistor between supply and base . thus calculations will be easier . i think you can predict how .

 

[Eshal]

Yes I can predict.. I am understanding..

 

[goldsmith]

So now is the time for simulating the result , do you know any thing about initial values of capacitors in pspice ?

 

[Eshal]

initial values... no I don't what do you mean by initial values of capacitor..

 

[goldsmith]

a capacitor for start oscillations need an initial values , in real world it will be prepared by circuit , but in simulations with Pspice , you have to add it to start analyzing . hence you will have many ways , such as IC probe , or perhaps two keys i have used . can you see what keys i'm referring to ? if yes , do you know how them doing their best ?

 

[Eshal]

Ni i don't know what two keys are you referring!!!
I don't understand exactly what does initial values mean.

 

[goldsmith]

Hi Eshal
Initial value ? consider you have an LC circuit . if you inject it ( tank circuit ) a low value current , it will give you a damped oscillations ( according to the value of L and C ) . for each analysis of such these circuits , you will need this injection ! ( initial value ) so , at Pspice , you have to add an instantaneous value , which can be added with two switch or a probe (ic ) . didn't you see keys such as that ? didn't you have circuit analysis in university ? or perhaps self reading in text books ?

 

[LvW]

I don't understand exactly what does initial values mean.

When using a capacitor in a circuit simulation program you, of course, can specify a certain value and - if you want - an "initial value (in volts)". That means: The transient simulation starts at t=0 under the assumption that the capacitor is charged already corresponding to the given initial voltage. (By the way - something similar is possible for an inductance L, but the initial value is a current).
Thus, no external switches or sources are necessary.

Remark: In PSpice the initial value is abbreviated as IC=

 

[Eshal]

Hi goldsmith!
Actually we had subject named "circuit analysis". We have 6 month a semester. Our teacher was too lazy or too fast, I don't know what should I say about her. She took 4 and half months for DC analysis and 1 and half month for AC. She covered DC part in detail but very fast, she had done cover 36 to 40 slides on projector in 40min class. So I assume you can imagine how our teacher teaches us at university. They consider us jackass and load us with lot of stuff for whole semester. We have no time to practical anything for our self.

Hi LvW!
Thank you. Now I am remembering yes we have studied first order circuit in which we have study initial response of capacitor and inductor.

BTW: LvW could make me understand about initial values.

- - - Updated - - -

Hi goldsmith!
Actually we had subject named "circuit analysis". We have 6 month a semester. Our teacher was too lazy or too fast, I don't know what should I say about her. She took 4 and half months for DC analysis and 1 and half month for AC. She covered DC part in detail but very fast, she had done cover 36 to 40 slides on projector in 40min class. So I assume you can imagine how our teacher teaches us at university. They consider us jackass and load us with lot of stuff for whole semester. We have no time to practical anything for our self.

Hi LvW!
Thank you. Now I am remembering yes we have studied first order circuit in which we have study initial response of capacitor and inductor.

BTW: LvW could make me understand about initial values.

 

[goldsmith]

Actually we had subject named "circuit analysis". We have 6 month a semester. Our teacher was too lazy or too fast, I don't know what should I say about her. She took 4 and half months for DC analysis and 1 and half month for AC. She covered DC part in detail but very fast, she had done cover 36 to 40 slides on projector in 40min class. So I assume you can imagine how our teacher teaches us at university. They consider us jackass and load us with lot of stuff for whole semester. We have no time to practical anything for our self.

Hi Eshal
An advise : if you want learn ! you shouldn't expect that your university teach you anything ! you should start to learn yourself ! and self reading ! it means be a bookworm ! and don't accept anything easily ! test , and see the result yourself with your hands and your eyes ! thus you can learn , and be able to be a designer ! most of the times i hate my professors in my university too ! because they are trying to teach things that they didn't understand ever ! never ! and of course when i'm in university in class i just can sleep or read my own books because of boring and wrong things ! so , i have learned all of things that i know with this way and helps of some of my best friends in this big world !
Anywhere :
Let's back to helping you :
So , now you know what are those keys , or perhaps Ic probe( as LvW corrected my answer ( and of course i improved my mind as his answer )) . can you simulate the oscillator that we have designed together for 100 KHZ ? ( now i have simulated it and it worked very fine ! )
Best Wishes
Goldsmith