Transcript Electronic Troubleshooting

Electronic Troubleshooting
Chapter 7
Transformer-Coupled Circuits
Transformer-Coupled Circuits
• Characteristics
• Some times used to couple stages of a circuit
• Offers some advantages
• When it is necessary to make either low or high impedances appear as
the opposite
• When it is desirable to only amplify a narrow band of frequencies
• Transformers transfer energy, thus: Pprimary = Psecondary
• Ideal ones match - real ones have some losses
• Aspects covered
•
•
•
•
•
Untuned Interstage Transformer Coupling
Transformer Coupled Amplifier
Test & Troubleshoot Transformer Coupled Amplifiers
Tuned Transformers
Amplifiers Working into Parallel-Tuned Circuits
Untuned Interstage Transformer
Coupling
• Characteristics
• Transformers designed to operated over a wide range of
frequencies are called untuned
• Example range of freqs – the audio spectrum 20 -20kHz
• Usually have lower losses than power transformer
• Example: Untuned 4:1 step down transformer
Untuned Interstage Transformer
Coupling
• Characteristics
• Example: Untuned 4:1 step down transformer
• n = Np/Ns = vp/vs
• n= turns ratio; Np= number of primary turns
• Ns= number of secondary turns; vp= primary voltage
• vs= secondary voltage
• If the secondary has less turns – Step-down transformer
• If the primary has less turns – Step-up transformer
• Example problem
• For the transformer on page 165
• Find the turns ratio and secondary voltage
Np
200
n

4
Ns
50
n
vp
vs
4
12v
 vs  3
vs
Untuned Interstage Transformer
Coupling
• Characteristics
• Secondary/Primary Current and Power
• is = vs /RL
• For the previous
example problem
is 
vs
3V

 200mA
RL 15
• Pprimary = Psecondary
i p  v p  is  v s
• For the previous
example problem
vs
i p  is 
vp
ip 
is
n
is 200mA
ip  
 50mA
n
4
Untuned Interstage Transformer
Coupling
• Characteristics
• Reflected Impedance
Zp 
•
vp
ip
Zp 
• For the previous
example problem
• Sub for ip
Zp 
vp
ip

vp
is
Zp 
n 
vp
ip

vp
vs
vp
is
n
vp
ip

12V
 240
50mA
is 
vs
RL
 nRL  n * nRL  n 2 RL  rreflected
Transformer Coupled Amplifier
• Key Aspects
• Amplifiers gain is dependent upon the load resistance
seen on the output. Distortion also is dependent.
• AV= rL/re
• Too A small load resistance
causes distortion
• Transformers can make a
small load appear to have
much higher resistance
rreflected  n RL
2
Transformer Coupled Amplifier
• Key Aspects
• Sample Circuit
• Given: re = 12Ω, Vin = 5mV
• Find: n, rref , AV , vL
rreflected  n RL
2
n
Np
Ns

150
6
25
rreflected  6 2  50
rreflected  1800
AV 
rL 1800

 150
re
12
vC  vi  AV  750mV
vp
750mV
n
6
 vs  125mV
vs
vs
Test & Troubleshoot Transformer
Coupled Amplifiers
• Key Aspects
• Typical collector Voltage
• Without signal
• Almost equal to Vcc
• Very low DC winding resistance
• Thus very small voltage drop on
the winding
• Goes higher than Vcc with input
• Inductive kickback
» Changing primary current causes
generation of voltage that adds to
the source voltage
» Can be very large with an open secondary
Test & Troubleshoot Transformer
Coupled Amplifiers
• Key Aspects
• Typical problems
• Open primary winding in the previous circuit
• No output at Vc
• Open Secondary
• 0V across the load
• High voltages across the primary (at the start also kickback)
• Shorted Secondary or Primary
• Reflected impedance near zero
• Almost no output AC signal – Dramatic drop in Av
• Transformer Replacement
• Use exact replacements if possible
• Else match the turns ratio and physical size – same size usually relates
to freq response characteristics
Tuned Transformers
• Key Aspects
• Covered Items
• Parallel Resonance
• Tuned transformers
• Parallel Resonance
• Characteristics
• Parallel circuit with and inductor and a Cap
» Often called a Tank Circuit
• At a Freq where XL = XC the circuit is at resonance
fr 
1
2 LC
Where :
f r  Re sonant  Freq
L  Coil  Induc tan ce  henries
C  Capaci tan ce  farads
• Resistance or Impedance of the Tank Circuit => Zt = Q XL
» Where Q = XL / Rc , XL =2πfL
Note the DC
resistance of
the Coil must
be less than
1/10 of XL at
resonance
Tuned Transformers
• Key Aspects
• Parallel Resonance
• Characteristics
• Resistance
» Curve to the right
• Equivalent of the inductor coil
» Has an ideal inductor in series
with a resister that = the DC
resistance of the inductor
• Example Problem
• Given: Circuit on the previous slide, L=2mH,
C=0.003µF, Rcoil = 20Ω
• Find: fr , Q, and Zt
fr 
1
2 LC

1
3
6.28  2 10  3 10
9
 64.9 KHz
Tuned Transformers
• Key Aspects
• Parallel Resonance
• Example Problem
• Given: Circuit on the previous slide, L=2mH,
C=0.003µF, Rcoil = 20Ω
• Find: fr , Q, and Zt
X L  2fL  6.28  64.9KHz  2mH  817
Q
X L 817

 40.8
Rc
20
Zt  Q  X L  40.8  817  33.4K
Tuned Transformers
• Key Aspects
• Tuned transformers
• Many coils can have their inductance adjusted in order to adjust the
resonant frequency
• They utilize threaded cylinders made of iron (called slugs)
• The amount of cylinder in the coil can be adjusted
• By changing the amount of Iron in the coil the inductance is
adjusted
» Thus the resonant frequency
• Adjustment of coil slugs should be minimized
• Usually can only be adjusted a
few times without damaging t
he coil
Tuned Transformers
• Typical Circuits
• Characteristics
• Like with untuned transformers a small load on the secondary is
reflected into the primary as a much higher impedance
• Thus increasing the gain of the driver
stage
• The gain curve with
respect to frequency looks
like the Impedance curve
on the previous slide
• Only a selected
small range of freq
are amplified to a
significant level
Tuned Transformers
• Typical Circuits
• Characteristics
• The gain curve with respect to frequency looks like the Impedance
curve on the previous slide
• For example the two IF amplifiers shown in Figure 7-7 on page
173 typically only have a bandwidth of 10KHz around 455KHz
• Bandwidth review
• Packaging – Varies. The ones used in the IF Amp on page 173
• Have the transformer and tank Cap in a grounded tin can
» Adjusting slug is accessed through a hole on the top
Tuned Transformers
• Typical Circuits
• Tuning process
• Varies per manufacturer
• Also called Alignment
• Some use O-Scopes others can use DC voltmeters on the AGC circuit
• Same end result – Tank resonant frequency is adjusted