BJT as an Amplifier

Hello dear students, I hope you all are doing great. In today’s tutorial, we will have a look at BJT as an Amplifier. A transistor is an electronic component that has three terminals. Terminals are base-emitter, collector and base. It used in different electronic projects and circuits for switching and amplification process.

Amplification is process through which weak input signal is amplified to a larger level. In today’s post, we will have a detailed look at the amplifier circuit using BJT. We will make a complete amplifier circuit and discuss all its parameters. So let’s get started with the BJT as an Amplifier.

BJT as an Amplifier

DC and AC Quantities

• Before an understanding of bjt amplifier circuit first, we must know about the designations used in the amplifier circuit for voltage current and resistor since in this amplifier circuitry ac and dc parameters are simultaneously used.

• Capital alphabets in an italic font will be used for both ac and dc current (I) and voltages (V).
• This principle is applicable to the peak, rms (root mean square), average and peak to peak ac parameters.
• The values used in circuitry for ac current and voltages are root mean squares until are mentioned for other types,
• Some parameters will use lowercase (i) and (v) for ac current and voltage, this lowercase text will use for only instantaneous quantities.
• DC values used in amplifier circuitry will have uppercase non-italic subscript.
• For instance, I_B, I_C, I_E are dc currents values of transistor and V_BE, V_CB, and V_CE are a value of dc voltage between any two terminals of the transistor.
• The value like V_B, V_C, and V_E that has single sub scripted voltage are voltage between terminal of transistor and ground.
• Ac and other variable parameters will have italic subscript.
• For instance, I_b, I_c and I_e are current values of the transistor.
• While V_be, V_cb, and V_ce are the ac voltage values between two terminals of the transistor.
• The voltage values that have single subscript are ac voltage between the transistor terminal and ground such as Vb, Vc, and Ve.
• The internal resistance of the transistor will have denoted by the lowercase having subscript.

• For instance, internal ac emitter resistance is denoted as r’e

• Circuit resistances exterior to the transistor itself use the italic capital R with a subscript that recognizes the resistance as dc or ac like the current and voltage.
• For instance, R_E is exterior dc emitter resistor and Re is an exterior ac emitter resistance.
• So friends I hope you have understands all parameters and terms used in the amplifier circuit.
• All these parameters will help to understand the circuit and equation used for amplifier circuitry.

BJT Voltage Amplification

• As we know that Ic=βI_b that mean current is amplified. In transistors, the value of base current is less ac compare to emitter and collector current so emitter and collector currents are almost equal.
• By keeping in mind this discussion lets have a look at this below given circuitry.

• You can see that ac voltage V_s is carried on the dc biased voltage VBB.
• The dc biased voltage V_CC is provided to the collector with the resistance R
• The ac input voltage generated ac base current due to that high ac collector current is generated.
• Due to ac collector current ac voltage generated about the resistance R_C, therefore creating an amplified, but inverted, generation of the ac input voltage in the active region of a transistor is shown in above figure.
• The base-emitter junction which is in forward biased condition provides less resistance to the ac signal.
• This interior ac emitter resistance r’_e is shown in the figure and in series with the R_B.
• The value of base ac voltage is given here.

V_b = I_er’_e

• The ac collector voltage Vc, is equal to the voltage loss across the resistance R_C.

Vc = IcRC

As Ic=Ie

• So collector voltage will be written as.

Vc =IeRC

• V_b can be supposed as the ac input voltage for transistor. Here Vb =Vs- IbRB.
• Vc can be taken as the ac output voltage of transistor.
• As the ratio between the output to input is known as voltage gain.
• So the ratio between Vc to Vb is ac voltage gain and denoted as Av.

Av=Vc/Vb

• If we put IeRC in place of Vc and I_er’_e for Vb than we have.

Av= IeRC/ I_er’_e

• By simplifying this equation we have.

Av= RC/ r’_e

• From this equation, you can see that the bjt used in circuitry gives amplification in the shape of voltage gain that is dependent on the values of RC and r’e.
• As RC is always significantly higher, the output voltage for this arrangement is larger than the input voltage.

So, friends, it is a complete post about BJT as an amplifier. If you have any questions about the BJT amplifier ask in comments. Thanks for reading.