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 4:1 step down transformer

4. 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

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 = 12Ω, 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 Note the DC resistance of the Coil must be less than 1/10 of XL at resonance • 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 => Zt = Q XL • Where Q = XL / Rc , XL =2πfL

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, L=2mH, C=0.003µF, Rcoil = 20Ω • Find: fr , Q, and Zt

13. 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

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