Black Box Electronics
An Introduction to Applied Electronics for Physicists
2. Analog Electronics: BJTs to opamps
University of Toronto
Quantum Optics Group
Alan Stummer, Research Lab Technologist
Active BJTs
Class A
with four resistors
Class A
with negative feedback
MOSFETs
(Metal Oxide Semiconductor Field Effect Transistor)
• Drain to source resistance and/or current ≈ 1 / gate to source voltage.
• Zero gate current!
• Operate in active region (analog), or saturation or cutoff (digital).
• Most are N-channel (source grounded, positive drain and gate).
• Limited in current ID, voltage VDS and power PD.
• Very good saturation, RDS(on).
• Limited frequency range, capacitance dominates at high frequency.
• Select for current, voltage, speed and RDS(on) if saturating.
• Typical voltage VDS range 10V to 200V, extreme to 1.5KV.
• Typical current ID range 100mA to 10A, extreme 10mA to 100A.
Sample FET Transfer Curves
A general purpose 48A, 60V N-FET from Fairchild (Digikey.ca).
http://www.fairchildsemi.com/ds/ND/NDP6060L.pdf
NDP6060L Data Sheet – Parametric Section
Basic Q Switching
NPN BJT
• Good speed.
• Poor saturation.
• Two components.
• Rule of thumb: Ib ≈ Ic/10
N-FET
• Good speed.
• Very good saturation.
• One component.
(Note that Id is 0.1% of limit yet it
works well)
Sample Linear FET Circuit
Opamp uses a N-FET to increase current driving capacity.
The opamp can supply only ten’s of mA but the laser needs ten times more.
1. The opamp can drive the FET from
cutoff through to saturation.
2. All of the laser current goes
through R9.
3. The opamp monitors the R9
voltage.
4. The opamp adjusts the FET gate
voltage to control the FET
conductance.
Less Common Active Parts
IGBT (Insulated Gate Bipolar Transistor)
• Current allowed through collector to emitter is proportional to voltage
between gate and emitter.
• Voltage controlled like a FET, switches like a BJT.
• Used for power control such as motors.
• Pros: High voltage and current. Cons: slow, poor saturation.
Thyristers: SCR (Silicon Controlled Rectifier) & Triac
• Anode/MT1 connects to cathode/MT2 once gate current exceeds
threshold, stays on until anode/MT1 current drops below threshold.
• SCR is unipolar, Triac is bipolar.
• Used for motor and lighting controls (Φ control).
• Pros: High voltage and current. Cons: slow, poor saturation.
OPAMPs
(Operational Amplifiers)
• Five terminal linear device (2 power, 2 inputs, one output).
• Hard Rule #1: The output goes positive if the input is positive
(where input is defined as +ve input relative to –ve input).
• Soft Rule #2: The inputs and output can only range between the “rails”
(the positive and negative supply pins).
• All else is imperfections:
• Finite open loop gain (GBW)
• Finite bandwidth (GBW)
• Limited output voltage range
• Limited output current
• Finite slew rate (output dV/dt)
• Unstable with capacitive load
• Noise generation
• Limited input voltage range
• Finite input current
• Finite input offsets
• Phase shifting (lower gain, stability)
• Finite quiescent supply current
• Temperature sensitivity and aging
of all parameters
Some OpAmp Errors
• Ib: Input bias current, ranges fA to µA.
• Io: Input offset current, ~10-50% of Ib.
• Vio: Input offset voltage, ranges 10’s µV to mV.
• Vn: Equivalent input noise, ranges 1-1000nV/√Hz
• Av: Open loop gain, usually 105-106.
• GBW, f: Gain bandwidth product, ranges 1KHz to 1GHz.
• Slew Rate, dV/Dt: Output slew rate, ranges 0.1V/µS to 10KV/µS.
• Iout: Output current limit, ranges 1mA to 10A.
• Plus temperatures, supply, load, Rin, drifts, aging, interactions,…
Basic OpAmp Amplifiers
• AV = -R1 / R2
• f-3dB = 1 / 2π R1 C1
• Vout = 1 + R3 / R4
• f-3dB = 1 / 2π R3 C2
In this example:
• Av = -100K / 10K = -10
• f-3dB = 1 / 2π * 100K * 1nF = 1.59KHz
• 0V < Vout < +5V
• Vin range depends on Vref
In this example:
• Av = 1 + 100K / 10K = +11
• f-3dB = 1 / 2π * 100K * 1nF = 1.59KHz
• 0V < Vout < +5V
• 0V < Vin < +5V
Ω The End Ω
Next:
3. Digital Electronics
4. Sample Circuits
5. Spice simulations
Then: More in depth on anything? Suggestions?