Question

QUESTION 2 You are attempting to implement a NOR gate by using the BJT circuit shown in Figure 2. Note that the two BJTs are identical. Vec 3V • VOLT R RS w A w B OL Figure 2: NOR gate implementation There are two operational requirements that you need to achieve: The required output voltage thresholds are: Von = 2.4 and VoL = 0.4. The current load at the base cannot exceed a certain value, i.e. Is s 1(max) = 100 A. Given that: • VC(rar) 0.2 v. Poc = 50, the input voltage (for both inputs at A and B) is V, - 1.5 V for HIGH level and V. - OV for LOW level, choose the values for R, and Rc from the list of standardised resistor values in Table 3 (Appendix A) to satisfy the operational requirements of the NOR gate. In your workings, clearly show that the chosen resistor values will be able to satisfy the operational requirements for each case (i.e. each row) presented in the truth table in Table 1 below. [12 marks] Table 1: NOR truth able A LOW LOW HIGH HIGH В LOW HIGH LOW HIGH F HIGH LOW LOW LOW Example: When we consider the first row of Table 1, we have VA = V- OV Vic. Thus, you have to show that Vour 2 24 V = Vow: Similarly, when we consider the second row of Table 1, you have to show that Vour S 0.4 V = Vou given that Vi = 0 V and Vj = 1.5V = VIH- Hint no.1: The output voltage thresholds are the upper/lower bounds for the voltage value, meaning the actual value of the output voltage can be lower (for LOW output) or higher (for HIGH output) than the corresponding threshold values. Hint no.2: Operate the BJTs either at cut-off or saturation

Answer 1

Given Veesat = 0.2 v Boc=50 &W, H = 1.5 for High. Voltage drop a cross Rc in Saturation = Vpc = Vec-Veesat. VRC = 3-0-2 = 2.8V. =) VOL = Ver-Vec = Veresat = 0.2V VOL = 02 <0.4V Vrc = Ic Rc =) Re= Vac Ic In Saturation Icc ßI & Isman 10oll A. Ic < 508 200MA = 5mA Iccsma =) Rc = VRc -> Re> Vec SMA 2.8 Ic - 0.56 K. SMA Re> 5602 when AGB VIH=1.5v VRB VI HUBE VIH RB + VRB = VIH-VBE VBE => IBRB = VH-VBE » IBRB = 1.5-VBE V Besut 20.7V Vae fy saturation IBRB € 1.5-0.7 IBRB < 0.8v 0.8 0.8 < Ro =) RB < Oogv IB 8km Ismer 100 MA RB <8km