pnp transistor

The PNP Transistor

The PNP Transistor is the exact opposite to the NPN Transistor device we looked at in the previous tutorial. Basically, in this type of transistor construction the two diodes are reversed with respect to the NPN type giving a Positive-Negative-Positive type of configuration, with the arrow which also defines the Emitter terminal this time pointing inwards in the transistor symbol.

Also, all the polarities for a PNP transistor are reversed which means that it “sinks” current into its Base as opposed to the NPN Transistor which “sources” current through its Base. The main difference between the two types of transistors is that holes are the more important carriers for PNP transistors, whereas electrons are the important carriers for NPN transistors.

Then, PNP transistors use a small base current and a negative base voltage to control a much larger emitter-collector current. In other words for a PNP transistor, the Emitter is more positive with respect to the Base and also with respect to the Collector.

The construction of a “PNP transistor” consists of two P-type semiconductor materials either side of an N-type material as shown below.

A PNP Transistor Configuration

(Note: Arrow defines the emitter and conventional current flow, “in” for a PNP transistor.)

 

The construction and terminal voltages for an NPN transistor are shown above. The PNP Transistor has very similar characteristics to their NPN bipolar cousins, except that the polarities (or biasing) of the current and voltage directions are reversed for any one of the possible three configurations looked at in the first tutorial, Common Base, Common Emitter and Common Collector.

PNP Transistor Connection

The voltage between the Base and Emitter ( VBE ), is now negative at the Base and positive at the Emitter because for a PNP transistor, the Base terminal is always biased negative with respect to the Emitter.

Also the Emitter supply voltage is positive with respect to the Collector ( VCE ). So for a PNP transistor to conduct the Emitter is always more positive with respect to both the Base and the Collector.

The voltage sources are connected to a PNP transistor are as shown. This time the Emitter is connected to the supply voltage VCC with the load resistor, RL which limits the maximum current flowing through the device connected to the Collector terminal. The Base voltage VB which is biased negative with respect to the Emitter and is connected to the Base resistor RB, which again is used to limit the maximum Base current.

To cause the Base current to flow in a PNP transistor the Base needs to be more negative than the Emitter (current must leave the base) by approx 0.7 volts for a silicon device or 0.3 volts for a germanium device with the formulas used to calculate the Base resistor, Base current or Collector current are the same as those used for an equivalent NPN transistor and is given as.

 

We can see that the fundamental differences between a NPN Transistor and a PNP Transistor is the proper biasing of the transistors junctions as the current directions and voltage polarities are always opposite to each other. So for the circuit above: Ic = Ie – Ib as current must leave the Base.

Generally, the PNP transistor can replace NPN transistors in most electronic circuits, the only difference is the polarities of the voltages, and the directions of the current flow. PNP transistors can also be used as switching devices and an example of a PNP transistor switch is shown below.

A PNP Transistor Circuit

 

The Output Characteristics Curves for a PNP transistor look very similar to those for an equivalent NPN transistor except that they are rotated by 180o to take account of the reverse polarity voltages and currents, (the currents flowing out of the Base and Collector in a PNP transistor are negative). The same dynamic load line can be drawn onto the I-V curves to find the PNP transistors operating points.

Transistor Matching

Complementary Transistors

You may think what is the point of having a PNP Transistor, when there are plenty of NPN Transistors available that can be used as an amplifier or solid-state switch?. Well, having two different types of transistors “PNP” and “NPN”, can be a great advantage when designing power amplifier circuits such as the Class B Amplifier.

Class-B amplifiers uses “Complementary” or “Matched Pair” (that is one PNP and one NPN connected together) transistors in its output stage or in reversible H-Bridge motor control circuits were we want to control the flow of current evenly through the motor in both directions.

A pair of corresponding NPN and PNP transistors with near identical characteristics to each other are calledComplementary Transistors for example, a TIP3055 (NPN transistor) and the TIP2955 (PNP transistor) are good examples of complementary or matched pair silicon power transistors. They both have a DC current gain, Beta, ( Ic/Ib ) matched to within 10% and high Collector current of about 15A making them ideal for general motor control or robotic applications.

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Also, class B amplifiers use complementary NPN and PNP in their power output stage design. The NPN transistor conducts for only the positive half of the signal while the PNP transistor conducts for negative half of the signal.

This allows the amplifier to drive the required power through the load loudspeaker in both directions at the stated nominal impedance and power resulting in an output current which is likely to be in the order of several amps shared evenly between the two complementary transistors.

Identifying the PNP Transistor

We saw in the first tutorial of this transistors section, that transistors are basically made up of two Diodes connected together back-to-back.

We can use this analogy to determine whether a transistor is of the PNP type or NPN type by testing its Resistance between the three different leads, Emitter, Base andCollector. By testing each pair of transistor leads in both directions with a multimeter will result in six tests in total with the expected resistance values in Ohm’s given below.

• 1. Emitter-Base Terminals – The Emitter to Base should act like a normal diode and conduct one way only.
• 2. Collector-Base Terminals – The Collector-Base junction should act like a normal diode and conduct one way only.
• 3. Emitter-Collector Terminals – The Emitter-Collector should not conduct in either direction.

Transistor Resistance Values for a PNP Transistor and a NPN Transistor

Between Transistor Terminals		PNP	NPN
Collector	Emitter	RHIGH	RHIGH
Collector	Base	RLOW	RHIGH
Emitter	Collector	RHIGH	RHIGH
Emitter	Base	RLOW	RHIGH
Base	Collector	RHIGH	RLOW
Base	Emitter	RHIGH	RLOW

 

Then we can define a PNP Transistor as being normally “OFF” but a small output current and negative voltage at its Base ( B ) relative to its Emitter ( E ) will turn it “ON” allowing a much large Emitter-Collector current to flow. PNP transistors conduct when Ve is much greater than Vc.

In other words, a Bipolar PNP Transistor will ONLY conduct if both the Base and Collector terminals are negative with respect to the Emitter

In the next tutorial about Bipolar Transistors instead of using the transistor as an amplifying device, we will look at the operation of the transistor in its saturation and cut-off regions when used as a solid-state switch. Bipolar transistor switches are used in many applications to switch a DC current “ON” or “OFF”, from LED’s which require only a few milliamps of switching current at low DC voltages, or motors and relays which may require higher currents at higher voltages.

current divider rule

current divider rule

In electronics, a current divider is a simple linear circuit that produces an output current (IX) that is a fraction of its input current (IT). Current division refers to the splitting of current between the branches of the divider. The currents in the various branches of such a circuit will always divide in such a way as to minimize the total energy expended.

The formula describing a current divider is similar in form to that for the voltage divider. However, the ratio describing current division places the impedance of the considered branches in the denominator, unlike voltage division where the considered impedance is in the numerator. This is because in current dividers, total energy expended is minimized, resulting in currents that go through paths of least impedance, therefore the inverse relationship with impedance. On the other hand, voltage divider is used to satisfy Kirchhoff's Voltage Law. The voltage around a loop must sum up to zero, so the voltage drops must be divided evenly in a direct relationship with the impedance.

To be specific, if two or more impedances are in parallel, the current that enters the combination will be split between them in inverse proportion to their impedances (according to Ohm's law). It also follows that if the impedances have the same value the current is split equally.

Current divider:

A general formula for the current IX in a resistor RX that is in parallel with a combination of other resistors of total resistance RT is (see Figure 1):

I_X = \frac{R_T}{(R_X)+(R_T)}I_T \ 

where IT is the total current entering the combined network of RX in parallel with RT. Notice that when RT is composed of a parallel combination of resistors, say R1, R2, ... etc., then the reciprocal of each resistor must be added to find the total resistance RT:

 \frac {1}{R_T} = \frac {1} {R_1} + \frac {1} {R_2} + \frac {1}{R_3} + ... \ . 

General case[edit]

Although the resistive divider is most common, the current divider may be made of frequency dependent impedances. In the general case the current IX is given by:

I_X = \frac{Z_T} {Z_X+Z_T}I_T \ ,[1]

Using Admittance[edit]

Instead of using impedances, the current divider rule can be applied just like the voltage divider rule if admittance (the inverse of impedance) is used.

I_X = \frac{Y_X} {Y_{Total}}I_T

Take care to note that YTotal is a straightforward addition, not the sum of the inverses inverted (as you would do for a standard parallel resistive network). For Figure 1, the current IX would be

I_X = \frac{Y_X} {Y_{Total}}I_T = \frac{\frac{1}{R_X}} {\frac{1}{R_X} + \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}}I_T

physics all equation

Mechanics

velocity

v̅ =  Δs Δt

v =  ds dt

acceleration

a̅ =  Δv Δt

a =  dv dt

equations of motion

v = v0 + at x = x0 + v0t + ½at2 v2 = v02 + 2a(x − x0) v̅ = ½(v + v0)

newton's 2nd law

∑ F = m a

∑ F =  dp dt

weight

W = m g

dry friction

ƒ = μN

centrip. accel.

ac =  v2 r

ac = − ω2 r

momentum

p = m v

impulse

J = F̅ Δt

J =  ⌠ ⌡ F dt

impulse-momentum

F̅ Δt = m Δv

⌠ ⌡ F dt = Δp

work

W = F̅Δs cos θ

W =  ⌠ ⌡ F · ds

work-energy

F̅Δs cos θ = ΔE

⌠ ⌡ F · ds = ΔE

kinetic energy

K = ½mv2

general p.e.

ΔU = −  ⌠ ⌡ F · ds

F = − ∇U

gravitational p.e.

ΔUg = mgΔh

efficiency

ℰ =  Wout Ein

power
P̅ =  ΔW Δt	P̅ = F̅v cos θ
P =  dW dt	P = F · v

angular velocity

ω̅ =  Δθ Δt

ω =  dθ dt

v = ω × r

angular acceleration

α̅ =  Δω Δt

α =  dω dt

a = α × r − ω2 r

equations of rotation

ω = ω0 + αt θ = θ0 + ω0t + ½αt2 ω2 = ω02 + 2α(θ − θ0) ω̅ = ½(ω + ω0)

2nd law for rotation

∑ τ = I α

∑ τ =  dL dt

torque

τ = rF sin θ

τ = r × F

moment of inertia

I = ∑ mr2

I =  ⌠ ⌡  r2 dm

rotational work

W = τ̅Δθ

W =  ⌠ ⌡  τ · dθ

rotational power

P = τω cos θ

P = τ · ω

rotational k.e.

K = ½Iω2

angular momentum

L = mrv sin θ L = r × p L = I ω

universal gravitation

Fg = −  Gm1m2  r̂ r2

gravitational field

g = −  Gm  r̂ r2

gravitational p.e.

Ug = −  Gm1m2 r

gravitational potential

Vg = −  Gm r

orbital speed

v = √  Gm r

escape speed

v = √  2Gm r

hooke's law

F = − k Δx

elastic p.e.

Us = ½kΔx2

s.h.o.

T = 2π √  m k

simple pendulum

T = 2π √  ℓ g

frequency

ƒ =  1 T

angular frequency

ω = 2πƒ

density

ρ =  m V

pressure

P =  F A

pressure in a fluid

P = P0 + ρgh

buoyancy

B = ρgVdisplaced

mass flow rate

I =  m t

volume flow rate

φ =  V t

mass continuity

ρ1A1v1 = ρ2A2v2

volume continuity

A1v1 = A2v2

bernoulli's equation
P1 + ρgy1 + ½ρv12 =	P2 + ρgy2 + ½ρv22

dynamic viscosity

η =  F̅/A Δvx/Δz

η =  F/A dvx/dz

kinematic viscosity

ν =  η ρ

aerodynamic drag

R = ½ρCAv2

mach number

Ma =  v c

reynolds number

Re =  ρvD η

froude number

Fr =  v √gℓ

young's modulus

F  = E  Δℓ A ℓ0

shear modulus

F  = G  Δx A y

bulk modulus

F  = K  ΔV A V0

surface tension

γ =  F ℓ

Thermal Physics

solid expansion

Δℓ = αℓ0ΔT ΔA = 2αA0ΔT ΔV = 3αV0ΔT

liquid expansion

ΔV = βV0ΔT

sensible heat

Q = mcΔT

latent heat

Q = mL

ideal gas law

PV = nRT

molecular constants

nR =Nk

maxwell-boltzmann

−  mv2 p(v) =  4v2 ⎛ ⎝ m ⎞3/2 ⎠ e 2kT √π 2kT

molecular k.e.

⟨K⟩ =  3  kT 2

molecular	speeds
vp = √  2kT m ⟨v⟩ = √8kTπm vrms = √  3kT m

heat flow rate

Φ̅ =  ΔQ Δt

Φ =  dQ dt

thermal conduction

Φ =  kAΔT ℓ

stefan-boltzmann law

Φ = εσA(T4 − T04)

wien displacement law

λmax =  b T

internal energy

ΔU = 3⁄2nRΔT

thermodynamic work

W = − ⌠ ⌡ P dV

1st law of thermo.

ΔU = Q + W

entropy

ΔS =  ΔQ T

S = k log w

efficiency

ℰreal = 1 −  QC QH

ℰideal = 1 −  TC TH

c.o.p.

COPreal =  QC QH − QC

COPideal =  TC TH − TC

Waves & Optics

periodic waves

v = ƒλ

frequency

ƒ =  1 T

beat frequency

fbeat = fhigh − flow

intensity

I =  ⟨P⟩ A

intensity level

LI = 10 log ⎛ ⎝ I ⎞ ⎠ I0

pressure level

LP = 20 log ⎛ ⎝ ∆Pmax ⎞ ⎠ ∆P0

interference fringes

nλ = d sin θ

nλ  ≈  x d L

index of refraction

n =  c v

snell's law

n1 sin θ1 = n2 sin θ2

critical angle

sin θc =  n2 n1

image location

1  =  1  +  1 ƒ do di

image size

M =  hi  =  di ho do

spherical mirrors

ƒ ≈  r 2

Electricity & Magnetism

coulomb's law

F = k  q1q2 r2

electric field, def.

E =  FE q

electric field, around charges
E = k ∑  q  r̂ r2	E = k  ⌠ ⌡ dq  r̂ r2

field and potential

E̅ = −  ∆V d

E = − ∇V

electric potential, def.

ΔV =  ΔUE q

electric potential, around charges
V = k ∑  q r	V = k  ⌠ ⌡ dq r

capacitance

C =  Q V

plate capacitor

C =  κε0A d

cylindrical capacitor

C =  2πκε0ℓ ln (b/a)

spherical capacitor

C =  4πκε0 (1/a) − (1/b)

capacitive p.e.

U =  1  CV2 =  1 Q2  =  1  QV 2 2 C 2

electric current

I̅ =  Δq Δt

I =  dq dt

ohm's law

V = IR E = ρ J J = σE

resitivity-conductivity

ρ =  1 σ

electric resistance

R =  ρℓ A

electric power

P = VI = I2R =  V2 R

resistors in series

Rs = ∑ Ri

resistors in parallel

1  = ∑  1 Rp Ri

capacitors in series

1  = ∑  1 Cs Ci

capacitors in parallel

Cp = ∑ Ci

magnetic force, charge

FB = qvB sin θ

FB = q v × B

magnetic force, current

FB = IℓB sin θ

dFB = I dℓ × B

biot-savart law

B =  μ0I ⌠ ⌡ ds × r̂ 4π r2

solenoid

B = µ0nI

straight wire

B =  μ0I 2πr

parallel wires

FB  =  μ0 I1I2 ℓ 2π r

electric flux

ΦE = EA cos θ

ΦE =  ⌠ ⌡ E · dA

magnetic flux

ΦB = BA cos θ

ΦB =  ⌠ ⌡ B · dA

motional emf

ℰ = Bℓv

induced emf

ℰ̅ = −  ΔΦB Δt

ℰ = −  dΦB dt

gauss's law

∯  E · dA =  Q ε0

∇ · E =  ρ ε0

no one's law

∯ B · dA = 0

∇ · B = 0

faraday's law

∮E · ds = −  dΦB dt

∇ × E = −  ∂B ∂t

ampere's law

∮B · ds = μ0ε0  dΦE  + μ0I dt

∇ × B = μ0ε0  ∂E  + μ0 J ∂t

Modern Physics

time dilation

t' =  t √(1 − v2/c2)

length contraction

ℓ' = ℓ √(1 − v2/c2)

relativistic mass

m' =  m √(1 − v2/c2)

relative velocity

u' =  u + v 1 + uv/c2

relativistic energy

E =  mc2 √(1 − v2/c2)

relativistic momentum

p =  mv √(1 − v2/c2)

energy-momentum

E2 = p2c2 + m02c4

mass-energy

E = mc2

relativistic doppler effect

λ  =  ƒ0  = √  ⎛ ⎝ 1 + v/c ⎞ ⎠ λ0 ƒ 1 − v/c

photon energy

E = hf

photoelectric effect

Kmax = E − ϕ = h(ƒ − ƒ0)

photon momentum

p =  h λ

schroedinger's	equation
iℏ  ∂  Ψ(r,t) = −  ℏ2  ∇2Ψ(r,t) + V(r)Ψ(r,t) ∂t 2m
Eψ(r) = −  ℏ2  ∇2ψ(r) + V(r)ψ(r) 2m

uncertainty principle

Δpx Δx ≥  ℏ  2

ΔE Δt ≥  ℏ  2

rydberg equation

1  = −R∞  ⎛ ⎝ 1  −  1 ⎞ ⎠ λ n2 n02

half life

N = N02−t/τ