Bypass Capacitor sets the gain and AC

[walters]

How does the Bypass Capacitor in parrallel with the Emitter Resistor
set the gain? the gain of what AC voltage or DC voltage?

Where does the AC come from because i'll i see from Emitter to Collector is
is DC voltage now AC so where is the AC voltage coming from ?

AC voltage from the Base doesn't go to the emitter or collector" what i was told

So where does the AC come from then if the Bypass Capacitor sets the gain?

 

[SkyHigh]

For more information, refer to Sedra & Smith, "Microelectronic Circuits" on small-signal amplification, single transistor amplifier, emitter degeneration.

A bypass capacitor in parallel with a emitter resistor Re sees its purpose in AC conditions. Capacitor in DC is an open-circuit. Thus static current Icq only flows throught emitter resistor in DC.

Icq sets the operating point of amplification of Class A and AB. However, you are amplifying AC signal, not DC current. BTW, DC is not a signal, as most people still do not know even after decades of work experience. So in AC, as capacitor has reactance Xc = 1/jwC, where w = 2 Pi f. Since all AC signals have frequencies, the higher the frequency, the lower the reactance Xc. Therefore it is technically assumable that in AC conditions, bypass capacitor short-circuited the emitter of transistor to ground. This is known as Emitter Degeneration because DC current bypasses Re via the bypass capacitor.

Since Vc / Vb = -Rc/Re, bypassing Re results Vc/Vb = -Rc/zero, resulting a very large or ideal AC voltage gain.

AC signal comes from any analogous sources like baseband voice or audio via an input transducer such as a microphone or temperature sensor. This AC signal in turn gets connected to the base of the BJT.

vb (AC signal at the base) modulates/controls/varies the vce (AC signal across collector-emitter) of transistor. Ve does not vary because it is the potential drop across Re. Vbe doesnt vary because it is the forward diode drop.

Notice I used big V for DC, small v for AC.

 

[walters]

So does the AC frequency signal go from BASE to Emiiter and BASE to collector?

 

[SkyHigh]

AC input signal at base goes to both collector and emitter. The difference is that the AC output signal at collector is 180° out-of-phase and Rc/Re times larger than input signal and the AC output signal at emitter is in-phase and same in amplitude as input signal.

 

[walters]

AC input signal at base goes to both collector and emitter

So the Transitor is a AC signal splitter than?

How does the Bypass Capacitor thats in parallel with the emitter resistor set the GAIN?

I mostly see a Bypass Capacitor in parallel with a emitter resistor to set the GAIN
but i don't understand how it sets the AC gain

 

[SkyHigh]

I have already explained. You have to read again

 

[walters]

Sorry Skyhigh i still don't get it i see that bypass capacitor in alot of gain circuits
for transitor or tube circuit preamps to add so more AC gain

AC signal pass through the bypass capacitor instead of the emitter resistor so it gets more Gain but how does it get more gain?

 

[SkyHigh]

Bypass capacitor short-circuited the emitter of transistor to ground. This is known as Emitter Degeneration because DC current bypasses Re via the bypass capacitor.

Since vb=ve and vc / ve = -Rc/Re, bypassing Re results vc/vb = vc/ve = -Rc/zero, resulting a very large or ideal AC voltage gain.

For greater details, read books that explain Emitter Degeneration.

 

[walters]

Bypass capacitor short-circuited the emitter of transistor to ground.

So does this Give more DC voltage to the Emitter lead to the transistor?

This is known as Emitter Degeneration because DC current bypasses Re via the bypass capacitor.

So the HFE in the transistor gets more GAIN from this?

 

[SkyHigh]

HFE has nothing to do with voltage, only DC current because HFE is DC current gain (sometimes also known as direct or forward current gain, depending the authors who wrote the books)

In AC condition, capacitor is no longer in open-circuit. It shorts the DC path from emitter to ground. No current flows through Re in AC condition.

There is neither DC voltage gain nor DC current gain. The entire principle is about AC voltage amplification, usually refer to as small-signal amplification.

 

[walters]

But why do they say in electronic books that AC flows through the bypass CAP to increase GAIN ? AC gain or DC gain?

 

[SkyHigh]

But why do they say in electronic books that AC flows through the bypass CAP to increase GAIN ? AC gain or DC gain?

In AC operation conditions.
Without the bypass capacitor, the AC voltage gain is -Rc/Re.
With the bypass capacitor, the AC voltage gain of -Rc/0, hence increased in AC voltage gain. This increase of AC voltage gain is the effect of emitter degeneration due to bypass capacitor shorting the path from emitter to ground.

 

[walters]

But the AC voltage is just going to ground its a shunt or short so how is that
GAIN to have your AC voltage go straight to ground?

 

[SkyHigh]

Does it matter? You are tapping the output signal from the collector, not from the emitter!

Please, go and read up more on what I have written or from textbooks! You obviously didn't read at all.

Added after 3 minutes:


What did I say before (several times repetitively)?---->

Without the bypass capacitor, the AC voltage gain is -Rc/Re.
With the bypass capacitor, the AC voltage gain of -Rc/0, hence increased in AC voltage gain

<--- Isn't this clear enough?[/quote]

 

[walters]

I think the bypass Capacitor just affects the BIASING of the transitor not the GAIN because tapping it off at the collector just getting the HFE from the transistor

 

[SkyHigh]

HFE varies very little due to the manufacturing process but bounded within range before shipped out to retails, in this case a BJT. How can this be ever affected by external components? I am really surprised.

Biasing is done at DC when you set the quiescent conditions to specify your operating point of amplification, for example, setting Icq.

The bypass capacitor is only intended for emitter degeneration to enhance AC or small-signal voltage gain. This capacitor shorts the emitter to ground in AC condition only.

I don't wish to repeat this anymore. If you still don't understand, I am really losing patience.

Read books written by Robert Boylestad & Nashelsky, Sedra & Smith, Gray & Meyer, Razavi, Allen & Holberg, etc. They will tell you the same thing like I did.

Added after 4 minutes:

Check that you are using a voltage-divider bias, BJT in common-emitter configuration. Rc is connected from Vcc to collector. Re in parallel with bypass capacitor. Two DC decoupling capacitors, Ci at the base node common to R1 and R2, Co at the collector node common to Rc. You tap the output signal at the other end of Co.

Added after 4 minutes:

Biasing can only be affected by R1, R2, Rc and Re.
Bypass capacitor should be a polarised capacitor with +ve electrode connected to emitter and -ve electrode to ground. Do not use a unpolarised capacitor.
For Decoupling capacitors, you can use polarised (with +ve electrode always to the input direction and -ve electrode to output direction) or non-polarised.

 

[walters]

Thanks alot for the information and time to write this stuff out

The bypass capacitor is only intended for emitter degeneration to enhance AC or small-signal voltage gain. This capacitor shorts the emitter to ground in AC condition only.

So if the capacitor shorts the emitter to ground does it Enhance the AC because there is no resistance in the emitter?
But the Transistor still has the same HFE gain inside the transistor so the small signal from the BASE of the transistor gets ampilifier more because the emitter of the transitor is shorted to ground by the capacitor the AC is just going to be shorted to ground so how did it enhance the AC?

Added after 1 hours 6 minutes:


When i put my (+) positve probe on the emitter where the bypass cap is and the emitter resistor is i don't read a Voltage GAIN increase why?

Only on the Colletor i read a voltage gain increase from the small signal from the input of the BASE transitor

 

[SkyHigh]

It enhances AC or small-signal voltage gain because in AC conditions, Re becomes "transparent", the bypass capacitor effectively presents a very low impedance between the emitter of the transistor and the ground. Hence there is only one path from emitter to ground and this path is established by the bypass capacitor, in AC conditions.

AC voltage gain = |-vc/vb| = |-vc/ve| = Rc/Re, without bypass capacitor. With the use of bypass capacitor, the this gain becomes Rc/0 or Rc/(close to zero), resulting an enhanced AC voltage gain as compared to Rc/Re (with bypass capacitor).

Only on the Colletor i read a voltage gain increase from the small signal from the input of the BASE transitor

This is precisely what you should obtain.
You detect by probing the output signal at the collector. The ratio between the output signal at the collector and the input signal at the base is effectively -Rc/Re. The '-' is because the output signal is 180° out-of-phase or inverted to the input signal.

When i put my (+) positve probe on the emitter where the bypass cap is and the emitter resistor is i don't read a Voltage GAIN increase why?

If you set the Digital Multimeter (DMM) to read AC voltage, you will read something close to 0V.
If you set the DMM to read DC voltage, you will read constant DC voltage.

Added after 1 minutes:

Correction

AC voltage gain = |-vc/vb| = |-vc/ve| = Rc/Re, WITHOUT bypass capacitor. WITH the use of bypass capacitor, the this gain becomes Rc/0 or Rc/(close to zero), resulting an enhanced AC voltage gain as compared to Rc/Re (WITHOUT bypass capacitor).

 

[walters]

Thanks alot for the information and time


I think i get it so the AC signal has a Bigger SWING because there is no emitter resistance to ground the AC just (SEE's") emitter going to ground and the collector resistor so it has a Bigger Swing so thats the Gain

The emitter resistor takes away from the AC signal limitng the SWING right?
without the cap there.

 

[SkyHigh]

Yes, you got it right. Congratulation!

The emitter resistor sets the lower limit where the output AC signal can swing. However with the use if a bypass capacitor, the lower limit is extended, thus giving the output signal a larger AC swing.

Cheers!