Compare TTL and CMOS logic level thresholds and input current characteristics.

• A. TTL typically has higher input impedance and thresholds around 0.5 Vdd.
• B. TTL typically has lower input impedance and a fixed threshold around 1.4 V; CMOS has higher input impedance with thresholds around 0.5 Vdd, and near-zero input current.
• C. CMOS thresholds are around 2.5 V for 5V supply and have high input current.
• D. Both TTL and CMOS share identical input impedance and thresholds.

Answer: (B)

The key idea here is how input impedance and switching thresholds differ between TTL and CMOS due to their internal transistor structures. TTL inputs use bipolar transistors that need base current, so their input impedance is relatively low and the input current is not near zero. The switching threshold for TTL is typically around 1.4 volts, sitting between the low and high input ranges (roughly 0.8 V and 2.0 V). CMOS, on the other hand, uses MOS transistors with gates that draw essentially no DC current, giving very high input impedance and near-zero input current. Its switching threshold tends to be around half of the supply voltage, so for a 5 V supply the threshold is about 2.5 V. So the best description is that TTL has lower input impedance with a fixed threshold around 1.4 V, while CMOS has higher input impedance with thresholds around 0.5 Vdd and near-zero input current. The other statements mix these characteristics, which isn’t accurate.