Basic Transistor Operation
- A bipolar junction transistor (BJT) is a semiconductor device with two pn junctions. In normal operation, the base-emitter junction (BE) is forward-biased, while the base-collector junction (BC) is reverse-biased.
- When base current (IB) is flowing across the BE junction and even though the BC junction is reverse-biased, then a current will flow from the emitter to the collector. This current is called the collector current (IC).
- The current flowing at the emitter is called the emitter current (IE).
- The relationship of these three currents is IE = IB + IC.
dc Transistor Circuit Analysis
- The ratio of the collector current IC to the base current IB is called the dc current gain or dc beta (DC). DC = IC/IE.
- Transistor data sheets usually refer to DC as hFE.
- The ratio of IC to IE is called the dc alpha (aDC). aDC = IC/IE
- VBE, when forward-biased, is about 0.7 V.
- Rearranging the equation for DC, we find that IC = DCIB. The collector current depends on the flow of the base current. The input current IB is amplified by the value of DC.
- The voltage drop across the collector resistor (RC) is VRC = ICRC.
- The voltage at the collector with respect to ground VCE is found by VCE = VCC VRC.
Cutoff and Saturation
- Cutoff is a condition in the operation of a BJT when IB = 0 and therefore IC = 0.
- During cutoff, the BE junction is reverse-biased, and only a small amount of collector leakage current (ICEO) flows.
- The BC junction is also reverse-biased during cutoff.
- Saturation is a condition when IE increases, causing IC to increase. This increase in ICE forces VCE to decrease to nearly zero. The BC junction becomes forward-biased, and VCE reaches a value called VCE(sat) (about 0.1 V).
- At saturation the collector current becomes known as IC(sat). Further increase in IB can cause no additional IC to flow.
- DC increases directly with an increase in temperature.
Maximum Transistor Ratings
- Manufacturers' data sheets list maximum parameters for safe operation of the BJT.
- These maximum ratings usually cover VCB, VCE, VBE, IC, and power dissipation PD(max).
- The product of IC and VCE must not exceed PD(max). Maximum values of both IC and VCE must not be used at the same time. Calculate these values with IC = PD(max)/VCE, or rearranging the equation, VCE = PD(max)/IC.
The Transistor as an Amplifier
- Amplification is the process of linearly increasing the amplitude of an electrical signal.
- Since IB is small compared to IC, an approximation can be made that IC = IE.
- Amplification most often deals with ac signals. The ac emitter current (Ie) = Vin/r ¢e where r ¢e is the ac resistance of the emitter.
- The ratio of Vout to Vin is called voltage gain (Av). Av = Vout/Vin.
- Voltage gain can be calculated by Av = RC/r ¢e.
The Transistor as a Switch
- Cutoff, the condition where IB = 0, also will cause VCE to equal VCC. In cutoff, then, the BJT is acting like an open switch. No current is flowing.
- Saturation, the condition where IC is maximum, causes VCE to be near zero. Since there is maximum IC, the internal resistance between the emitter and the collector is very low. In saturation, the BJT is acting like a closed switch. Maximum current is flowing.