# Junction Capacitances of Transistor at RF Frequencies

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#### WTech

##### Newbie level 5 I am stuck in designing amplifier circuit using BJT at 1GHz. I need to calculate its junction capacitances at above frequency and biasing point selected by me, using given in datasheet.
can any body help me

Thanks

#### Abhishekabs Why you are calculating junction capacitances??
You get everything in datasheet required to design an amplifier, if it is not sufficient then contact manufacturer for more data.

#### WTech

##### Newbie level 5 data sheet only contain capacitance at specific junction voltage and specified frequency, and it changes with respect to voltage and frequency. so i need to know how can i calculate it, using given capacitance in datasheet at my own frequency and junction voltages.

#### Abhishekabs You need to serach for equeations in books relatated to microwave transistors.
also see books such as solid state devices by Ben G Streetman.

#### timof WTech said:
data sheet only contain capacitance at specific junction voltage and specified frequency, and it changes with respect to voltage and frequency. so i need to know how can i calculate it, using given capacitance in datasheet at my own frequency and junction voltages.
There is no simple formula for voltage dependence of capacitance for arbitrary (i.e. arbitrary doping profile) p-n junction. There are analytical formulas for p-n junctions with constant doping profiles and for linearly graded profiles, but real p-n junctions may show a difference voltage behavior. (as an example, for p-n junction with constant doping profile, capacitance scales as C~1/sqrt(V-Vbi) with voltage V (Vbi is built-in voltage for p-n junction).

The "intrinsic" capacitance of p-n junction (that is, the capacitance of the depleted region) is independent of frequency, assuming an instantaneous response of the quasi-neutral regions to applied voltages. In reality, if doping (or conductivity) of the quasi-neutral regions is not high enough, the effective capacitance becomes frequency dependent, as there is a series resistance (of the quasi-neutral regions) in series with the "intrinsic" capacitance - so you get an RC effects and associated frequency dispersion of capacitance. If current flow in p-n junction (in quasi-neutral regions) is lateral , i.e. parallel to the plane of the p-n junction, you may need to consider a distributed RC network/model.

Max
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#### FvM

##### Super Moderator
Staff member data sheet only contain capacitance at specific junction voltage and specified frequency, and it changes with respect to voltage and frequency
You are misunderstanding the datasheet, I think. Usually capacitances are specified with a measuring frequency (1 MHz)
and a voltage. Additionally, most datasheets also have a diagram of the voltage depency. If not present, you can assume the
voltage depency to be similar to other transistors of same technology, that have this data.

But what does the 1 MHz mean? In my opinion, it's simply a sufficient low frequency to observe a pure capacitance.
The capacitance is almost frequency independant, but at higher frequencies, it doesn't appear as a pure capacitance, you have to use
e.g. S-parameters to describe the transistor impedances. At 1 GHz, also package impedances, particularly inductances play
an important role, they are also considered with S-parameters. Either you have S-parameter tables from the manufacturer, or
you can try a SPICE simulation, but it's only meaningful with a correct package model.

#### VSWR Which transistor are you using (designation, manufacturer)?

WTech said:
I am stuck in designing amplifier circuit using BJT at 1GHz. I need to calculate its junction capacitances at above frequency and biasing point selected by me, using given in datasheet.
can any body help me

Thanks

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