Electronic Troubleshooting

1
Electronic Troubleshooting

• Chapter 7
• Transformer-Coupled Circuits

2
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

3
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 41 step down transformer

4
Untuned Interstage Transformer Coupling

• Characteristics
• Example Untuned 41 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

5
Untuned Interstage Transformer Coupling

• Characteristics
• Secondary/Primary Current and Power
• is vs /RL
• For the previous
• example problem
• Pprimary Psecondary
• For the previous
• example problem

6
Untuned Interstage Transformer Coupling

• Characteristics
• Reflected Impedance
• For the previous
• example problem
• Sub for ip

7
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

8
Transformer Coupled Amplifier

• Key Aspects
• Sample Circuit
• Given re 12?, Vin 5mV
• Find n, rref , AV , vL

9
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

10
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

11
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
• Resistance or Impedance of the Tank Circuit gt Zt
  Q XL
• Where Q XL / Rc , XL 2pfL

Note the DC resistance of the Coil must be less
than 1/10 of XL at resonance
12
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, L2mH,
• C0.003µF, Rcoil 20?
• Find fr , Q, and Zt

13
Tuned Transformers

• Key Aspects
• Parallel Resonance
• Example Problem
• Given Circuit on the previous slide, L2mH,
• C0.003µF, Rcoil 20?
• Find fr , Q, and Zt

14
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

15
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

16
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

17
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