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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 2fL 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