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Electronic Circuits I > Test 2 > Flashcards

Test 2 Flashcards

(64 cards)

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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
Large signal model - conventional current always flows from a positive supply toward lower potential
NPN —> C to E | PNP —> E to C
26
Large signal model - distinction between active and saturation regions based on the BC Junction bias
NPN —> collector voltage not lower than base voltage PNP —> collector voltage must be higher than base voltage
27
Bipolar Amplifiers: at the input
Circuit must operate as a voltmeter Ideal impedance = infinity Output remains open because it is not connected to any external sources
28
Bipolar Amplifiers: at the output
Circuit must behave as a voltage source Ideal impedance = 0 Input shorted to represent 0 voltage
29
Bipolar Amplifiers: Biasing - 2 objectives
Ensure operation in the forward active region Set Ic to the value required in the application
30
Bipolar Amplifiers: Simple Biasing
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
Bipolar Amplifiers: Simple Biasing - disadvantages
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
Bipolar Amplifiers: Resistive Divider Biasing
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
Bipolar Amplifiers: Biasing with Emitter Degeneration
Alleviates the problem of sensitivity to beta and Vbe
34
Bipolar Amplifiers: Biasing with Emitter Degeneration - Re
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
Bipolar Amplifiers: Biasing with Emitter Degeneration - I1 >> Ib
To lower sensitivity in beta
36
Bipolar Amplifiers: Biasing with Emitter Degeneration - Vre must be large enough
100mV to several hundred mV To suppress the effect of uncertainties in Vx and Vbe
37
Bipolar Amplifiers: Biasing with Emitter Degeneration - Design procedure
``` 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
Bipolar Amplifiers: Biasing with Emitter Degeneration - overly conservative design problems
If I1 >> 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
Bipolar Amplifiers: Self-Biased Stage
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
Bipolar Amplifiers: Self-Biased Stage - important guidelines for design
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
Bipolar Amplifiers: Self-Biased Stage - design procedure
Calculate Ic using gm Calculate Vbe Find Rc then Rb
42
Common Emitter Topology
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
Common Emitter Topology: Analysis of CE Core
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
Common Emitter Topology: Analysis of CE Core - design
Ic — > assume value for Vbe —> Rc —> voltage gain
45
Intrinsic gain
No external device loads the circuit Represents the maximum voltage gain provided by a signal transistor Independent of bias current
46
Common Emitter Topology: CE Stage with Emitter Degeneration
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
Common Emitter Topology: CE Stage with Emitter Degeneration - adding a capacitor
If C is very large, acts as a short circuit
48
Common Emitter Topology: CE Stage with Emitter Degeneration - adding Rb
Only degrades the performance but often proves inevitable (scaled down by beta+1)
49
Coupling capacitor
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
Output impedance >> load impedance
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
Use of capacitor to eliminate degeneration
Lowers gain but stabilizes bias point despite beta and Is Av = -gmRc Rin = r(pi)||R1||R2 Rout = Rc
52
Use of capacitor to eliminate degeneration: design
``` Re Ce Rc Vbe then Vx then R1+R2 using 10Ib If Rin low, use 5Ib ```
53
CE stage with Rs and Rl
Lowers voltage gain
54
Common Base Topology
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
Emitter Follower
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
Emitter Follower with Rs
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
Voltage amplifiers
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
Impedance looking into the base
Rpi
59
Impedance looking into collector
Ro
60
Impedance looking into emitter
1/gm
61
CE stage provides
Moderate voltage gain, input impedance, and output impedance
62
Emitter degeneration pros
Improves linearity but lowers voltage gain Raises output impedance of CE stages
63
CB stage provides
Moderate voltage gain, low input impedance, moderate output impedance
64
Emitter follower provides
High input impedance, lower output impedance, voltage gain less than 1

Electronic Circuits I (3 decks)

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