Current mirror

1
Current mirror

• Current mirror is a programmable current source
• What is the load current (IC2)?

15V
IC2
IC1
2
Twin - matched pair

• Q1 and Q2 are matched pair (TWIN)
• they have exactly the same performance because
  they are fabricated on the same chip, at the same
  temperature and have the same level of doping
• Since Q1 and Q2 are matched, and have the same
  VBE
• IC2 IC1
• We can use the resistor in Q1 to control IC2
• IC2 IC1 (15V - 0.6V) / 15kW 1mA

3
Current mirror with ratios other than 11

• Quiz What is the current ratio? Why?

4
Current mirror with ratios other than 11

• Quiz What is the current ratio? Why?

5
Temperature effects

• IC increase with the temperature
• if VBE remains constant,
• IC grows at about 9 / oC
• if IC remains constant,
• VBE falls at 2mV / oC
• a smaller VBE is enough to provide the same IC
  because of the higher temperature

6
Temperature effects

• The great thing about mirror is that IC2 is not
  affected by temperature change !
• If we heat up Q1, will IC1 be changed?
• VBE can only vary from 0.5 to 0.8V
• IC remains more or less fixed at 1mA

7
Negative feedback

• A deeper reason why IC1 is constant
• temperature gt IC1
• IC1 gt voltage drop across 15k resistor
• gt VBE1
• gt IC1
• This feedback mechanism keeps IC1 constant

8
Temperature compensation for amplifier

• If VBE is fixed, what would happen if we heat up
  the transistors in the following circuits?

9
Auto-bias with feedback

• Left-hand circuit (without feedback resistor RE)
• extremely sensitive to temperature change
• IC at a rate of 9/oC, bias changed
• Right-hand circuit (with feedback resistor RE)
• temp , IC , VE , VBE
  (as VB is fixed),
• sqeezing VBE so that IC
• Another reason why IC is constant

10

• The price to pay
• lower gain ( -RC/RE) in exchange for stability
• Can we have both stability (constant DC bias) and
  high (ac) gain?
• Yes !!

11
Requirements of output driver

• High current output
• need large current to drive the mechanical voice
  coil of the speakers driver
• Voltage gain is not important
• because signal has already been amplified
• An emitter follower is perfect
• VIN VOUT
• small current in (IB), large current out (IE)
• Power out gt power in gt a power amplifier

12
Output driver circuit

• A driver that produces large current over range
  /-15V

13
Output driver circuit

• Q1 and Q2 are both emitter followers
• What is the use of the current source in Q1?
• To replace RE
• Problem with RE
• If RE is too small gt small input impedance
• If RE is too large gt small IC gt small output
  impedance
• Active load (the current source)
• it has impedance gt large input impedance
• You choose large IC so that output impedance is
  small

14
Use a constant current source instead of resistor

• Q2 is a power transistor, needs large base
  current
• large resistor gives only small current
• ideal constant current source has infinitely
  large impedance and can provide large current at
  the same time
• Clever !
• Current source is an example of active load
• Used extensively in integrated circuits because
  it is easier to fabricate transistors than
  resistors

15
Output driver circuit

• DC offset voltage
• 0.6V (the VBE) for a simple follower
• Zero DC offset voltage in this circuit !
• Q1 (pnp) and Q2 (npn) are opposite type of
  transistors
• Note the 0.6V voltage drops of VBE in Q1 and Q2
  cancels each other out
• So that 0V DC input gives 0V DC output

16
Current source and current sink

• Current source
• The ability of giving out current (flow out of
  the circuit)
• Current sink
• The ability of absorbing current (flow into the
  circuit)
• Consider Q2
• Range of VE /- 15V
• If VE gt 0, current flowing out to load (source)
• If VE lt 0, current flowing in from load (sink)

17
Current source and current sink

• Max source current (when Q2 is fully on)
• If Q2 is fully ON (VCE0)
• max source current 15V/8W 2A
• Max sink current (when Q2 is fully off)
• IR ISINK IE
• ISINK is max when IE 0 (zero current from Q2)
• this happens when VE -15V
• Max ISINK 15 / 8 A

18
Max output power of the amplifier

• Max output voltage swing
• /- 15V peak-to-peak
• Average power dissipated on a 8W load

19
Root-mean-square (RMS) voltage

• Define RMS voltage as
• Advantage
• For ac voltage, the average power
• Hong Kongs power supply 240V
• It is a RMS voltage

20
Quiescent (rest state, no signal) power
dissipation

• At rest state, 0V input gives 0V output
• power dissipated in resistor (0- -30)2/8
  110W !!
• power dissipated in transistor Q2
• quiescent collector current 30V/8 3.75A
• VCE 15V, IC 3.75A,
• power VCE IC 56W
• total quiescent power dissipation 110W56W
• 
  166W (HOT !!)
• 14W amplifier gives out 166W heat (inefficient)

21
Push-pull emitter follower (Class B amplifier)

• Problem with class A amplifier
• quiescent power dissipation is many times larger
  than maximum output power
• push-pull follower

22
Class B amplifier

• Q1 (NPN) conducts on positive cycle
• Q2 (PNP) conducts on negative cycle
• 0V input, none of the transistor is conducting
• No current, no power dissipation at Q1 and Q2
• No resistor, no power dissipation
• Pros
• Efficient, no wasted power
• Cons
• Cross-over distortion

23
Cross-over distortion

• input have to be greater than 0.6V or less than
  -0.6V in order to make Q1 or Q2 conducting
• Output 0V when input is between 0.6V and -0.6V
• Distortion

24
Class AB amplifier

• Use diode to compensate for crossover distortion
• R can be replaced by current source

0.6V
25
Amplifiers classification

• Class A transistors conduct all the time
  (360o)
• Class B transistors conduct only half of the
  time (180o)
• Class AB transistors conduct more than half of
  the time, and less than full time
• Class C transistors conduct less than half of
  the time

26
Differential Amplifier

• A differential amplifier has two input terminals
• only amplify the difference in signals
• common signal is NOT amplified

27
Differential Amplifier

• The pair of input signals can be separated into a
  common part and a differential part
• e.g. (3V, 3.1V) inputs (0.05V, -0.05V)
  (differential)
• (3.05V, 3.05V) (common)
• Ideal differential amplifier
• Large differential gain
• Amplify the difference in signal
• Zero common-mode gain
• common signal produces zero output

28
Long-tail pair
29
DC Quiescent point

• 0V input (because base is tied to ground)
• VA -0.6V
• ITAIL
• ITAILis pretty constant because
• large negative voltage (-15V)
• variation of VA is small in practice
• A simple constant current source
• Can be improved by using a real current source
• By symmetry (same VBE), IC on both transistors is
  about 1mA each

30
Differential gain

• The key in understanding differential gain
• feed a DVIN/2 signal on one input, and a -DVIN/2
  on the other, VA does not move

VA
DVIN/2
-DVIN/2
31
Differential gain

• Because VA is fixed, only need to analyze the
  part of the circuit above A
• just a common emitter amplifier

32
Differential gain

• Common emitter amplifier gain
• VOUT/VIN -RC/(REre)
• In out case, input -DVIN/2
• Differential gain

33
Common mode gain

• Feed a common signal to both inputs, VA goes up
• model the 7.5k tail
• by two 15k in parallel
• I 0 because of symmetry
• split the circuit in two
• Common mode gain
• GCM -RC/ (reRE2(RTAIL))

Vin
34
CMRR (Common-mode rejection ratio)

• CMRR differential gain / common mode gain
• Ideal differential amp has
• large differential gain
• small common mode gain
• Therefore CMRR should be as large as possible
• RTAIL should be as large as possible

35
CMRR

• Replace RTAILby constant current source
• CMRR 100,0001 !!
• 100dB !
• dB 20log 105
• 205 100

36
How to generate composite signal in the lab

• Composite signal common-mode differential
  signal
• use function generator 1 to generate the
  common-mode signal (tied to true ground)
• use function generator 2 to generate the
  differential signal (floating ground, ground is
  connected to the output of generator 1)
• Make sure the ground of generator 2 is floating
  (not connected to the main supply)

37
Generator 2
Common signal
Generator 1
38
Vdiff Vcommon
Floating ground !
Vcommon
Vcommon (from gen 1)