# IF amplifier troubles :/

Discussion in 'The Projects Forum' started by yoamocuy, Oct 26, 2010.

1. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
Hello, I am building an intermediate frequency amplifier for a class project and I've run into a bit of a problem. This IF amplifier is supposed to be built for 455 kHz with a 10 kHz bandwidth. For this project I can only use transistors and IF transformers or filters. Also, my circuit has to be powered by a 9 volt battery. I'm in a group and we are building an AM receiver as a whole so I will be receiving a small AM signal. I'll go ahead and show what I've done so far and then state what my problem is.

At the moment I'm just working on the amplification portion of the 1st stage IF amplifier. Once I get the amplification working right, I plan on adding an IF transformer to both the input and output, as is shown in the paint edit, and adjusting the internal impedance to get the required bandwidth.

I will now go through the calcs I've done to get the values shown in my document labeled multisim. I have an inductance of 480uH and a Capacitance of 254.9 pF at the collector because 2*∏*f=1/√(L*C). Using these values gave me a f of 455 kHz which is what I need. I later hooked up a bode plotter and saw that the peak of my bode plot was at 455 kHz as it should have been.

When I originally designed this circuit, I had it constructed in such a way that the AC output was over 10V pk to pk. After talking to my professor, I decided to make the output 9V pk to pk. To do this, I realized I would need my Q point to be at the center so I first chose an Re of 300Ω and then calculated my needed Icq to be 12.85 mA. I did this using the equation Vcc=Ic*(Re+Rc)+Vce. As Vce goes to 0, the circuit is in saturation, my Vcc was 9V, my Re was 300Ω, and I said that my Rc was 50Ω because of the internal resistance of the inductor.

The β of my transistor is 155, which I edited in multisim, so I found my needed ibq to be 82.9 uA. I made a voltage loop and came up with the equation Ibq=(Vbth-Vbe)/(Rbth+hie+Re*(β+1)), where Vbth is the equivalent Thevenin voltage and Rbth is the equivalent Thevenin resistance. I solved for hie using the equation hie=Vt/Icq*β, where Vt is 25mV. This gave me a value of hie to be 301. I then chose a value of Vbth to be 4.7V and found the Rbth to be 1,150Ω. By using the equations Rbth=R1||R2 and Vbth=Vcc*R2/(R1+R2), I found R1 to be 2200Ω and R2 to be 2400Ω.

To find my RL, I used the equation VceAC=Vce+Icq*RAC. I was thinking that RAC would be just my RL, but I'm now wondering if I perhaps need to include some different internal resistance from my inductor capacitor couple since they are at resonance. Considering my RL to be the only AC resistance, I found it to be 350Ω.

The big problem that I foresee at the moment, is that when I simulate in multisim, my output voltage gradually increases and never seems to cease increasing. I'm not sure why this is.

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
Over what time scale are you running the simulation?

Common Emitter Intermediate Frequency amplifier stages normally have a primary-tapped / slug-tuned transformer in the collector branch - to reduce collector loading and provide inter-stage isolation.

Have you researched typical IF amplifier stages?

I ran your circuit in my simulation software [transient mode] and did not observe any problems - there was a front end transient (expected) which vanished in short order.

3. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
I'm not sure on the time scale, I'll have to check tomorrow when I go back by the computer labs.

I've researched a decent amount, but am still looking around a bit as I continue to change my circuit a bit. I'm going to be hooking up some stuff this weekend and looking at voltages at different frequencies and such. Thanks for your help.

4. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
Typically the B+ to the transistor collector is run through the primary of the interstage transformer. Check out some of the older transistor radio schematics that should be easy enough to google up.

5. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
After doing some research I changed my circuit design a bit. I couldnt get an If transformer in multisim so I just took it to the lab. I've hooked everything up but I'm not getting any output :/. I tried putting a 470Ω resistor at the output and then looking at the voltage across that resistor also but still didnt see anything. I can get amplification without the if transformer by using a LC couple in it's place and putting a resistor at the load but the problem with that is that I can't set the bandwidth. I can make the cutoff frequency at 455 kHz but the bandwidth is massive. With the transformer in place, I'm not exactly sure how to calculate what other resistors I should be using. I just calcualted them the same way I did before assuming 50Ω internal resistance. I know the transfomer has an internal impedance, but is it's total impedance still 50k even though it's in parallel with the coils?

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6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Instead of the virtual transistor, pick an actual part number out for better simulation results.

When this circuit is going to be assembled, I don't think there will be a BJT_NPN_VIRTUAL in the parts drawer.

7. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
Ok, I can do that but how can I get an IF transformer? I tried just putting a regular transformer and placing a capacitor across pins 1 and 3 to make it an IF transformer but I can't figure out how to change the turns ratio.

8. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
720
Model for IF transformer