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Electronic Circuits I > Test 2 > Flashcards

Test 2 Flashcards

1
Q

NPN Transistor: Structure

A

Two pn junction diodes

  • BE Forward Biased (Vbe > 0)
  • CB Reverse Biased (Vc > Vb and Vbc < 0)
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2
Q

NPN Transistor: operation in active mode

A

Acts as a voltage controlled current source

Base region very thin

Cannot be modeled as two back-to-back diodes

Carries a large number of electrons from E, through B, to C while drawing a small current of holes through base terminal

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3
Q

How do electrons travel through the base?

A

Diffusion

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4
Q

NPN Transistor: Base-Emitter Junction

A

Electrons flow from E to B
Holes flow from B to E

More electrons than holes (E doping level greater than base, n+)

E injects a large number of electrons into the base while receiving a small number of holes

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5
Q

NPN Transistor: what happens to electrons as they enter the base?

A

Since base region is thin, most of the electrons reach the edge of the collector-base depletion region, beginning to experience the built-in electric field

Electrons are swept into collector region and absorbed by the positive battery terminal

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6
Q

NPN Transistor: collector-base junction

A

Carries a current because minority carriers are injected into its depletion region

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7
Q

NPN Transistor: base region

A

Small electric field (allows most of the field to drop across BE depletion layer)

Drift current is negligible

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8
Q

NPN Transistor: collector current

A

Does not depend on collector voltage in active mode

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9
Q

NPN Transistor: base current

A

Results from the flow of holes

As the electrons injected by E travel through B, some may “recombine” with the holes

Must supply holes for both reverse injection into E and recombination with the electrons traveling toward C

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10
Q

NPN Transistor: large-signal model

A

Diode between B and E

Voltage-controlled current source between C and E

Chain of dependencies: Vbe —> Ic —> Ib—> Ie

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11
Q

NPN Transistor: I/V Characteristics

A

Ic vs. Vbe with Vce constant
-exponential relationship (acts like a diode)

Ic vs. Vce with Vbe constant

  • moves up and down with different values of Vbe
  • horizontal line because Ic is constant if in active mode (Vce > Vbe)
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12
Q

NPN Transistor: transconductance

A

Tells us about the performance of the device

As Ic increases, the transistor becomes a better amplifying device by producing larger collector current excursions in response to a given signal level applied between B and E

A function of collector current (if Ic constant, gm constant)

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13
Q

NPN Transistor: small signal model

A

Make small changes in Vce or Vbe and observe the changes in Ic, Ib, and Ie

With a high collector bias current, a greater gm is obtained, but the impedance between B and E falls to lower values

VCC must be replaced with a zero voltage to signify the zero change (ground supply voltage)

Voltages with no change replaced with a ground connection

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14
Q

NPN Transistor: Early Effect

A

If Rc increases, so does the voltage gain of the circuit

Translates to nonideality in the device that can limit the gain of amplifiers

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15
Q

NPN Transistor: Early Effect - Increasing Vce

A

Widening depletion region in C and B areas

Base charge profile must fall to zero at the edge of depletion region, so the slope increases

Base width decreases, increasing collector current

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16
Q

NPN Transistor: Early Effect - I/V Characteristics

A

Ic vs. Vbe

  • remains exponential
  • greater slope

Ic vs. Vce

  • non zero slope (Ic/Va)
  • Vce &laquo_space;Va
  • this variation reveals that the transistor does not operate as an ideal current source, requiring modification
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17
Q

NPN Transistor: Early Effect - small signal model

A

Collector current does vary with Vce (ro - output resistance)

Gain is eventually limited by the transistor output resistance

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18
Q

NPN Transistor: Operation in Saturation Mode - Vce approaches Vbe

A

Vbc goes from a negative value towards zero

BC junction experiences less reverse bias

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19
Q

NPN Transistor: Operation in Saturation Mode - Vce = Vbe

A

BC junction sustains a zero voltage difference

Depletion region still absorbs most of the electrons injected by E into B

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20
Q

NPN Transistor: Operation in Saturation Mode - “saturation region”

A

Vce < Vbe; Vbc > 0; BC junction FB

Collector voltage drops, BC junction experiences greater FB, carrying a significant current

Large number of holes must be supplied to base terminal
-leads to sharp rise in base current and rapid fall in beta

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21
Q

NPN Transistor: Operation in Saturation Mode - soft saturation

A

Diode sustaining small forward bias with extremely small current but still operates in active mode (Vbc < 400mV)

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22
Q

NPN Transistor: Operation in Saturation Mode - I/V Characteristics

A

Net Ic decreases as the device enters saturation because part of the controlled current is provided by the BC diode and need not flow from the collector terminal

Ic vs. Vce
-Ic falls for Vce less than V1

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23
Q

PNP Transistor: Operation

A

Emitter heavily doped (p+)

Active region

  • BE Junction : FB (Vbe < 0)
  • BC Junction : RB (Vbc > 0)
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24
Q

PNP Transistor: Active Mode

A

Majority carries in E (holes) are injected into B and swept away into C

Linear profile of holes formed in B to allow diffusion

Small number of base majority carriers (electrons) injected into E or recombined with holes in B, creating the base current

Base and collector voltage lower than emitter voltages

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25
Q

Large signal model - conventional current always flows from a positive supply toward lower potential

A

NPN —> C to E

PNP —> E to C

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26
Q

Large signal model - distinction between active and saturation regions based on the BC Junction bias

A

NPN —> collector voltage not lower than base voltage

PNP —> collector voltage must be higher than base voltage

27
Q

Bipolar Amplifiers: at the input

A

Circuit must operate as a voltmeter

Ideal impedance = infinity

Output remains open because it is not connected to any external sources

28
Q

Bipolar Amplifiers: at the output

A

Circuit must behave as a voltage source

Ideal impedance = 0

Input shorted to represent 0 voltage

29
Q

Bipolar Amplifiers: Biasing - 2 objectives

A

Ensure operation in the forward active region

Set Ic to the value required in the application

30
Q

Bipolar Amplifiers: Simple Biasing

A

Base tied to VCC through a large Rb, so as to FB BE junction

Calculation of Vce necessary as it reveals whether the device operates in the active mode or not

To avoid saturation completely: Vce > Vbe

Operating at the edge of active and saturation modes: Vce = Vbe

Ib —> Ic —> Vce

31
Q

Bipolar Amplifiers: Simple Biasing - disadvantages

A

More sensitive to Vbe variations among transistors or with temperature

If beta increases from 100 to 120, then Ic rises and Vce falls, driving the transistor to heavy saturation

32
Q

Bipolar Amplifiers: Resistive Divider Biasing

A

Ib not negligible —> replace voltage divider with a Thevenin equivalent

Vbe —> Ib —> Ic

Exponential dependence of Ic upon the voltage generated by the resistive divider still leads to substantial bias variations

1% error in one resistor values introduces a 36% error in Ic

33
Q

Bipolar Amplifiers: Biasing with Emitter Degeneration

A

Alleviates the problem of sensitivity to beta and Vbe

34
Q

Bipolar Amplifiers: Biasing with Emitter Degeneration - Re

A

Resistor Re in series with emitter, thereby lowering sensitivity to Vbe

  • occurs because Re exhibits a linear I-V relationship
  • an error in VX due to inaccuracies in R1, R2, or VCC is partly absorbed by Re, introducing a small error in Vbe and hence Ic
35
Q

Bipolar Amplifiers: Biasing with Emitter Degeneration - I1&raquo_space; Ib

A

To lower sensitivity in beta

36
Q

Bipolar Amplifiers: Biasing with Emitter Degeneration - Vre must be large enough

A

100mV to several hundred mV

To suppress the effect of uncertainties in Vx and Vbe

37
Q

Bipolar Amplifiers: Biasing with Emitter Degeneration - Design procedure

A 
Use gm to find Ic
Find Vbe
Assume Vre = 200mV and find Re
Find R1+R2 using 10Ib
Find R2 then R1
Find Rc
38
Q

Bipolar Amplifiers: Biasing with Emitter Degeneration - overly conservative design problems

A

If I1&raquo_space; Ib, then R1+R2 are quite small, leading to a low input impedance

If Vre large, then Vx (=Vbe+Vre) must be high, thereby limiting the minimum value of the collector voltage to avoid saturation (Rc smaller)

39
Q

Bipolar Amplifiers: Self-Biased Stage

A

Called “self-biased” because base current and voltage are provided from the collector

Vb always lower than Vc

  • guarantees that it operates in active mode
  • if Rc increases indefinitely, transistor remains in active region
40
Q

Bipolar Amplifiers: Self-Biased Stage - important guidelines for design

A

VCC-Vbe must be much greater than the uncertainties in the value of Vbe

Rc must be much greater than Rb/beta to lower sensitivity to beta

41
Q

Bipolar Amplifiers: Self-Biased Stage - design procedure

A

Calculate Ic using gm
Calculate Vbe
Find Rc then Rb

42
Q

Common Emitter Topology

A

Input —> base
Output —> collector
Emitter terminal grounded

Small increment of deltaV applied to base increases Ic by gm(deltaV) and hence the voltage drop across Rc by gm(deltaV)(Rc)

43
Q

Common Emitter Topology: Analysis of CE Core

A

Small signal gain negative because raising Vb and hence Ic lowers Vout

Gain proportional to gm and Rc

Input impedance decreases as collector bias increases

Output impedance trades with voltage gain

Rc fixed, voltage gain increased by increasing Ic, lowering both the voltage headroom and the input impedance

Early effect limits the voltage gain even if Rc approaches infinity

44
Q

Common Emitter Topology: Analysis of CE Core - design

A

Ic — > assume value for Vbe —> Rc —> voltage gain

45
Q

Intrinsic gain

A

No external device loads the circuit

Represents the maximum voltage gain provided by a signal transistor

Independent of bias current

46
Q

Common Emitter Topology: CE Stage with Emitter Degeneration

A

Improves linearity of the circuit

Voltage gain of the degenerated state lower than that of the CE core with no degeneration
-reduction in gain incurred to improve other aspects of the performance

Increases input impedance of the CE stage

47
Q

Common Emitter Topology: CE Stage with Emitter Degeneration - adding a capacitor

A

If C is very large, acts as a short circuit

48
Q

Common Emitter Topology: CE Stage with Emitter Degeneration - adding Rb

A

Only degrades the performance but often proves inevitable (scaled down by beta+1)

49
Q

Coupling capacitor

A

Used to isolate the bias conditions from undesirable effects

Bias point of Q remains independent of the resistance because C carries no bias current

Value chosen so that it provides a low impedance (almost a short circuit)

50
Q

Output impedance&raquo_space; load impedance

A

Connection of the load to the amplifier drops gain

Fix with voltage divider circuit and capacitive coupling

Rin = r(pi)||R1||R2
Rout = Rc||ro
51
Q

Use of capacitor to eliminate degeneration

A

Lowers gain but stabilizes bias point despite beta and Is

Av = -gmRc

Rin = r(pi)||R1||R2

Rout = Rc

52
Q

Use of capacitor to eliminate degeneration: design

A 
Re
Ce
Rc
Vbe then Vx then R1+R2 using 10Ib
If Rin low, use 5Ib
53
Q

CE stage with Rs and Rl

A

Lowers voltage gain

54
Q

Common Base Topology

A

Input —> emitter
Output —> collector
Base grounded

If Vin goes up by a small amount deltaV, the base emitter voltage decreases by the same amount because Vb is fixed. Ic falls by gm(deltaV), allowing Vout to rise by gm(deltaV)(Rc)

55
Q

Emitter Follower

A

Input —> base
Output —> emitter
Collector grounded

If Vin rises by a small amount deltaVin, Vbe tends to increase, raising Ie and Ic. Higher Ie —> higher Vout

Vout always lower than Vin by an amount equal to Vbe —> level shift

Change in Vout cannot be large than change in Vin

  • if the output changes by a greater amount than the input, then Vbe2 < Vbe1
  • decreases Ie and Vout

Gain <= 1

Acts as a voltage divider

56
Q

Emitter Follower with Rs

A

Transforms Rs to a much lower value, providing higher “driving” capability

Operates as a good “voltage buffer” because it displays a high input impedance (voltmeter) and a low output impedance (voltage source)

Input and output depend on the load and source impedances

57
Q

Voltage amplifiers

A

Must ideally provide a high input impedance (to sense a voltage without disturbing the node) and a low output impedance (to drive a load without reduction in gain)

58
Q

Impedance looking into the base

59
Q

Impedance looking into collector

60
Q

Impedance looking into emitter

61
Q

CE stage provides

A

Moderate voltage gain, input impedance, and output impedance

62
Q

Emitter degeneration pros

A

Improves linearity but lowers voltage gain

Raises output impedance of CE stages

63
Q

CB stage provides

A

Moderate voltage gain, low input impedance, moderate output impedance

64
Q

Emitter follower provides

A

High input impedance, lower output impedance, voltage gain less than 1

Electronic Circuits I (3 decks)

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