Transistor Logic (ttl)

Transistor - Transistor Logic (TTL)

• iR

• iC3

• vi

• vo

• TTL basics of operation
• Similar to that of simplified TTL
• Why Q2 and Q4 added?
• Why the diode D added?
• For input low,
• Current iR flows out the input (E of Q1) into the output of inverter driving input.
• Q2 is in cutoff since it gets no base current from Q1.
• Q3 is in cutoff since it gets no base current from Q2.
• Output is high since iC3 ≈ 0.
• For input high,
• Current iR flows out C of Q1 into base of Q2.
• Q2 comes on in active mode and then moves into saturation.
• Q2’s emitter current provides base current for Q3.
• Q3 comes on in active and then moves into saturation mode so output goes low, i.e. vo ≈ VCE,sat = 0.2V.

Transistor - Transistor Logic (TTL)

• Assumptions for analysis:
• All transistors are npn and identical.
• For p-n junctions in transistors to be conducting current,
• VBE or VBC must be 0.7V or higher.
• For smaller junction voltages, current flow is negligibly small.
• For transistors in forward active mode,
• iC = β iB,
• VBE,active ≈ 0.7 V
• For transistors in forward saturation mode,
• VBE,sat ≈ 0.8 V.
• VCE,sat ≈ 0.2 V.
• iC < β iB or iC / iB < β.
• For transistors in inverse mode (E junction reverse biased, C junction forward biased),
• current gain is very small
• iE = βr iB
• βr is very small, e.g. βr ~ 0.02

• vi

• vo