# Common Base Amplifier's Base Capacitor

#### SamR

Joined Mar 19, 2019
4,913
Most of the Common Base Amplifiers include a capacitor from the transistor base to ground. Some do not. Just what is the purpose of this capacitor? I built this circuit for example, which does not include the capacitor. Adding or removing a base capacitor seems to have no effect on the amplifiers output. Any AC component on the base will be going to ground through R2 even with a capacitor added. Is the capacitor intended to be a low resistance path to ground for any AC component?

#### crutschow

Joined Mar 14, 2008
33,358
No you likely won't see any affect at such a low frequency, but you may at higher frequencies (e.g. above 1MHz).
The base capacitor is intended to keep the transistor base at AC ground, independent of the frequency.
The base-emitter capacitance can affect this at higher frequencies.

#### Bordodynov

Joined May 20, 2015
3,117
See

#### Danko

Joined Nov 22, 2017
1,774
One more demonstration:

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#### Jony130

Joined Feb 17, 2009
5,475
Adding or removing a base capacitor seems to have no effect on the amplifiers output.
I highly disagree with you. The difference is huge in this case. Danko's simulation shows this very well.

Without a CB capacitor, the base terminal is no longer at GND for the AC signal.
Thus, the voltage gain will drop due to the negative feedback effect caused by the "reflected into emitter side" base resistance seen from the base terminal to AC ground.
And this resistance seen from the base terminal into the AC signal ground is equal to RB = 100kΩ||470kΩ ≈ 82.5kΩ.
Also, Rin will also be increased.

So without a CB capacitor. The voltage gain will be equal to:

$A_V \approx \frac{R_C||R_L}{\frac{R_B}{\beta +1} + r_e} \approx \frac{5k\Omega}{\frac{82.5k\Omega}{100} + 34.8\Omega}\approx 5.8 \: [V/V]$

But if you add a CB capacitor with a properly selected value (Xc << RB for AC signal). The voltage gain will be increased to this value:

$A_V \approx \frac{R_C||R_L}{r_e} \approx 143\: [V/V]$

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#### SamR

Joined Mar 19, 2019
4,913
I ran the frequency of my test circuit up and saw what you were telling me. Without the capacitor it quit fairly quickly and with the capacitor the breadboard gave out ~1MHz. Thanks guys!

#### crutschow

Joined Mar 14, 2008
33,358
I ran the frequency of my test circuit up and saw what you were telling me. Without the capacitor it quit fairly quickly and with the capacitor the breadboard gave out ~1MHz.
So why did you initially see no change with and without the capacitor?

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#### SamR

Joined Mar 19, 2019
4,913
I've looked at the rsube model of the common base configuration and although I follow the logic of its analysis it doesn't go into frequency response. But looking at the model that @Bordodynov provided, the output voltage increases with frequency. From the equations @Jony130 provided without the base capacitor the Av gain calculation gives a lower gain result which results in lower amplitude outputs without the base capacitor. Although I can see the results, I do not have an innate understanding of the exact methodology. Hopefully that will come with more experience...

#### Bordodynov

Joined May 20, 2015
3,117
Your source circuit is not a common-base circuit. Common base means that the base is AC grounded, and you don't have that. A more correct definition is when both the signal source and load are connected to the base. This can be achieved by using capacitors whose impedance is low over the operating frequency range. Since you used 1kHz, the capacitances were not large enough, resulting in a reduction in gain.