1. Logistics

  • DBM 1 Canvas submission available

2. BJT modes recap

bjt rc re sim sch
Figure 1. LTspice schematic
bjt rc re sim plot
Figure 2. LTspice results
LTspice hot tips

The value of resistor RC is defined using curly braces {rc}
→ define (.param) and reference ({x}) variables in LTspice.

The simulation does a DC sweep (.dc) and then changes the value of RC and re-runs the simulation (.step)
→ for loops!

3. Rapid Analysis Objectives

  • Eyeball a circuit and guess the easiest places to begin analyzing.

  • Assumptions ?!

    • What to assume because including more precision doesn’t change numbers much.

    • “Aggressive Ohm’s Law” — avoid using any (or very few) transistor specifics until you’ve found the resistor currents.

    • Ignoring base current is usually a good first step

      • unless a small current at that node makes a difference

  • You simply need to be fast at circuit analysis fundamentals.  At least in setting up the equations.  Actually solving them can be tedious.

  • Do you really need the solution to this particular equation?

  • KEEP YOUR EYE ON THE PRIZE

    • This is not abstract art interpretation: “There is a chair in the painting” is what you should observe first --- the obvious stuff.

    • Analyzing something is done for a reason (rarely for the pure joy of the process).  Forget the reason and you can lose your way amidst the details.[1]

3.1. AoE Fig 2.9, p.76 LED driver circuit

aoe fig2 9

  • What is the LED current? (assume a red LED with Vf = 1.8 V)

  • What is the base current?


According to the schematic, Q1 is meant to function like a:

  • boss

  • switch

  • (linear) amplifier

4. Rapid circuit analysis exercise

weed eater
Figure 3. A circuit to practice fast analysis

Quickly find all node voltages and both collector currents in this circuit.

Where do you start?[2]

5. QRP Labs QMX+ radio

https://qrp-labs.com/qmxp.html

QRPlabs-QMXplus_schematics_rev2.pdf

qmxp power in

There is a pushbutton switch that can connect PWR_ON to GND which the user presses to initially turn on the device. PWR_HOLD is an MCU output pin that (especially when there is no power) is initially at 0 V. PWM_5V also starts at 0 V

  • Combination of Q103 and Q105

  • Effect of grounding PWR_ON node

  • Releasing PWR_ON when PWR_HOLD is low

  • Releasing PWR_ON if PWR_HOLD is 3.3 V

1. There are occasions when you really do need the exact number, or symbolic solution, or answer.  When you get to that point, you know a) that you need it, b) why you need it, c) how the nature of the answer affects the rest of the problem.  When finished, you have likely just made a legit technological breakthrough.  Make sure to document the process, the solution, and the context for why you needed it.  Then enjoy a brief moment of intense satisfaction before getting back to filing your TPS reports.
2. This is named the "`weed eater circuit`". Search for it to read about its origin.