Deck 2: Bipolar Junction Transistors BJTS

Question 1

Figure 6.1.1
The BJT in the circuit in Fig. 6.1.1 has $\beta=100$ . Find $V_{E}, V_{C}$ , and $I_{B}$ for the three cases:
(a) $V_{B}=0 \mathrm{~V}$ , (b) $V_{B}=3 \mathrm{~V}$ , and (c) $V_{B}=5 \mathrm{~V}$ .

Answer 1

(a)


Figure 6.1.2(a)
The solution is illustrated in Fig. 6.1.2(a). The transistor is cut off with
$$V_{E}=0 \mathrm{~V}, \quad V_{C}=+10 \mathrm{~V}, \quad I_{B}=0 \mathrm{~mA}$$
(b)


The solution is illustrated in Fig. 6.1.2(b). The transistor is operating in the active region with the following results:
$V_{E}=+2.3 \mathrm{~V}, \quad V_{C}=+4.3 \mathrm{~V}, \quad I_{B}=0.01 \mathrm{~mA}$
(c)



The analysis performed in Fig. 6.1.2(c) shows that the transistor cannot be operating in the active region. Thus, it must be saturated. The corresponding analysis is shown in Fig. 6.1.2(d) with the following results:
$V_{E}=+4.3 \mathrm{~V}, \quad V_{C}=+4.6 \mathrm{~V}, \quad I_{B}=1.07 \mathrm{~mA}$

Question 2

(a)The BJTs in the circuits of Fig. 6.2.1 have $\beta=100$ . (a) For the circuit in Fig. 6.2.1(a), find $V_{E}, I_{B}$ , and $V_{C}$ .
(b) For the circuit in Fig. 6.2.1(b), find $I_{B}, I_{C}$ , and $V_{C}$ .

Answer 2

(a)


Figure 6.2.2(a)
The analysis is illustrated in Fig. 6.2.2(a). We have assumed that $Q$ is operating in the active mode, and the results obtained justify this assumption: $V_{C}$ is lower than $V_{B}$ , keeping the CBJ reverse biased. The results are
$V_{E}=+0.7 \mathrm{~V}, \quad I_{B}=0.01 \mathrm{~mA}, \quad V_{C}=-2.03 \mathrm{~V}$
(b)


$Q$ cannot be in Active Mode; must be in Saturation
Figure 6.2.2(b)
The analysis in Fig. 6.2.2(b) is based on the assumption that the transistor is operating in the active mode. The results obtained show that this assumption is not justified: $V_{C}$ at $-425 \mathrm{~V}$ implies that the CBJ has a forward bias of $425.7 \mathrm{~V}$ ! Thus, the transistor must be in saturation. The corresponding analysis is shown in Fig. 6.2.2(c) and the results are
$I_{B}=0.43 \mathrm{~mA}, \quad I_{C}=0.47 \mathrm{~mA}, \quad V_{C}=+0.3 \mathrm{~V}$

Question 3

The transistor in the circuit of Fig. 6.3.1 has $\beta=$ 100. Find the values of the four resistors so that $V_{B}=+5 \mathrm{~V}, I_{1}=0.1 \mathrm{~mA}, I_{E}=1 \mathrm{~mA}$ , and $V_{C}=$ $+6 \mathrm{~V}$ .

Answer 3

The complete solution is performed directly on the circuit diagram in Fig. 6.3.2. The solution is based on the fact that the transistor is operating in the active mode because $V_{C}>V_{B}$ . The results are
$$\begin{aligned}R_{B 1} & =100 \mathrm{k} \Omega, & & R_{B 2}=55.6 \mathrm{k} \Omega, \\R_{E} & =4.3 \mathrm{k} \Omega, & & R_{C}=9.1 \mathrm{k} \Omega\end{aligned}$$

Question 4

Figure 6.4.1
The BJTs in the circuit of Fig. 6.4.1 have $\beta=100$ and $\left|V_{C E \text { sat }}\right|=0.3 \mathrm{~V}$ . Find $v_{O}$ for each of the following cases:
(a) $v_{I}=0 \mathrm{~V}$
(b) $v_{I}=+5 \mathrm{~V}$
(c) $v_{I}=-5 \mathrm{~V}$
(d) $v_{I}=+10 \mathrm{~V}$
(e) $v_{I}=-10 \mathrm{~V}$

Question 5

Figure 6.5.1
Design the circuit in Fig. 6.5.1 to obtain $I_{1}=$ $0.1 \mathrm{~mA}, V_{B 1}=+4 \mathrm{~V}, I_{E 1}=1 \mathrm{~mA}, V_{C 1}=+6 \mathrm{~V}$ , $I_{E 2}=2 \mathrm{~mA}$ , and $V_{C 2}=+4 \mathrm{~V}$ . Let $\beta_{1}=\beta_{2}=100$ and $\left|V_{B E}\right|=0.7 \mathrm{~V}$ . Specify the required values of all resistors.

Question 6

The BJT in the circuit in Fig. 6.6.1 has $I_{S}=$ $10^{-15} \mathrm{~A}$ at room temperature.
(a) Find $V_{O}$ at room temperature where $V_{T}$ is $25 \mathrm{mV}$ .
(b) If the temperature increases by $50^{\circ} \mathrm{C}$ , what will $V_{O}$ become?
(c) If, alternatively, the temperature decreases by $50^{\circ} \mathrm{C}$ , what will $V_{O}$ become?