BJT polarity swap: When a pnp transistor in a circuit is replaced by an npn transistor for analysis, how should previously calculated polarities and directions be adapted to analyse the new circuit correctly?

Introduction / Context:Transistor circuits built with pnp and npn devices are duals of each other. The small-signal and large-signal equations are identical in form if the reference polarities and current directions are consistently reversed. This question checks the core idea behind converting a solved pnp biasing/problem into the corresponding npn case without redoing every derivation from scratch.

Given Data / Assumptions:

• The original circuit used a pnp transistor and was fully analysed.
• The new circuit uses an npn transistor in the same topology.
• Device models follow conventional sign conventions: for npn, base–emitter forward bias is approximately +0.7 V; for pnp it is approximately −0.7 V with reversed polarities and current directions.

Concept / Approach:

An npn behaves like a pnp with all terminal voltages and currents inverted with respect to the chosen reference. If a pnp analysis gave certain node voltages and branch currents, the npn solution can be inferred by reversing the assumed polarities (voltage drops) and the assumed current directions while keeping magnitudes consistent. This is an application of network duality and sign convention consistency, not a change of the underlying transistor laws.

Step-by-Step Solution:

Verification / Alternative check:

Re-derive one KVL around the input loop and one KCL at the collector node. You will recover the same magnitude relations with flipped signs, confirming that polarity and direction reversals are sufficient.

Why Other Options Are Wrong:

• Repeating all calculations wastes effort; symmetry allows reuse.
• Flipping only voltages or only currents violates KCL/KVL consistency.
• Flipping supply rails alone is incomplete without reversing reference directions.

Common Pitfalls:

• Keeping the same VBE sign between pnp and npn.
• Forgetting to reverse the sign of node voltages referenced to ground.

Final Answer:

Replace all previously calculated voltages and currents by their reverse polarities and directions.