Square wave into a transformer, low impedance output needed

In summary, the 1:1 transformer will not work because the seen impedance will be 5k when the bjt is off, which will not satisfy the load requirements. An emitter follower off of the collector may solve the problem. There are no standard improvements, but depending on the load, complementary output stages with NPN and PNP transistors may work well.
  • #1
gnurf
370
8
I need to connect the transformer to an 5V open-collector source as shown below, but the load must not see a larger impedance than 50Ω. The 5V source is an internal source to the driver and not available outside of it. As it stands, the seen impedance will be 5k when the bjt is off, so this will not work. I need something in between and was thinking of an emitter follower (or maybe a opamp buffer), but my simulations went a bit awry (struggling with my transformer model) so I thought I'd throw this out here in case there is an obvious solution (which there probably is) that I've missed. Any ideas are welcome.

zM5UY.png
 
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  • #2
gnurf said:
I need to connect the transformer to an 5V open-collector source as shown below, but the load must not see a larger impedance than 50Ω. The 5V source is an internal source to the driver and not available outside of it. As it stands, the seen impedance will be 5k when the bjt is off, so this will not work. I need something in between and was thinking of an emitter follower (or maybe a opamp buffer), but my simulations went a bit awry (struggling with my transformer model) so I thought I'd throw this out here in case there is an obvious solution (which there probably is) that I've missed. Any ideas are welcome.

zM5UY.png

Yes, I'd use an emitter follower off of the collector. And you need to move the DC blocking capacitor from the output/secondary to the input/primary...
 
  • #3
On second thought, the CE output can only pull up on the output, so there is nothing to pull down for the other half-cycle. You should probably use a complementary output stage with NPN and PNP transistors...
 
  • #4
Thanks for the input. I've played around in spice and this is what I got so far:

ddR58.png

K9JKh.png


As the circuit stands the impedance the transformer (1:1) sees will be < 50 Ω and thus the load on the secondary side will be satisfied on that account. Additional load requirements are transition time < 100ns and TTL voltage levels. I'm not happy with how delicate the balance between sharp edges (low values of R1 and R2) and enough voltage headroom on the output (large R1 R2 give amplitude closer to 5V than low values of R1, R2). Any quick fixes for this problem, or is this a tuning problem I have to live with?

Any favorite work-horse transistors/diodes that could work well under the above conditions?

Are there any standard improvements to this circuit?

What kind of load is the 1:1 transformer? Will it affect performance (assuming it has a bandwidth of 400MHz)?
 
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1. What is a square wave and how does it affect a transformer?

A square wave is a type of electrical signal that alternates between two voltage levels at a consistent rate. When this type of signal is applied to a transformer, it can cause fluctuations in the magnetic field, which can lead to distortions in the output signal.

2. Why would a low impedance output be needed for a square wave into a transformer?

A low impedance output is needed for a square wave into a transformer because it allows for better control of the current flow and reduces the risk of signal distortion. This is especially important for high-frequency square waves, as they can cause more severe distortions without a low impedance output.

3. How does the impedance of the transformer affect the square wave?

The impedance of the transformer determines how much resistance the transformer will have to the flow of current. If the impedance is too high, it can cause the square wave to be distorted and affect the output signal. On the other hand, a low impedance can result in a cleaner and more accurate output signal.

4. Can a transformer handle a square wave with high voltage levels?

Yes, a transformer can handle a square wave with high voltage levels. However, it is important to ensure that the transformer is designed to handle the specific voltage levels needed for the square wave. Using a transformer with a lower voltage rating can result in damage and distortions in the output signal.

5. What are some applications for using a square wave into a transformer with a low impedance output?

Square waves are commonly used in electronic circuits for switching and timing applications. They are also used in power supplies to convert AC electricity into DC electricity. Therefore, a transformer with a low impedance output would be useful in these applications to ensure a clean and accurate output signal.

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