Buffer amp for 50 ohm line

Hello Mark,

Is the end of the 50 Ohm cable terminated in 50 Ohms, or some other resistance value? When a cable is referred to as 50 Ohm cable that is the surge impedance, or characteristic impedance of the line. That impedance is approximately the SQRT L/C, where L is the distributed inductance and C is the distributed capacitance of the line. In high frequency and above applications the line needs to be terminated in its characteristic impedance to prevent standing waves along the cable. They create voltage maximas and minimas along the line. However, if you are using at low frequencies the cable is so short relative to a wavelength no appreciable standing waves are created. Therefore, you don't necessarily have to terminate the cable with a 50 Ohm load.

Assuming you want to drive a 50 Ohm load at the end of the cable then indeed an amplifier with high output current would be needed. The BUF634, in conjunction with an op amp would easily drive a 50 Ohm load. The electrometer op amp would be your precision, ultra-low input bias current op amp configured with the BUF634 which would supply the 100 mA output current. The BUF634 operating current can be programmed with a resistor for more or less current, and more or less bandwidth.

If you do chose to leave the cable unterminated its capacitance will be presented to the op amp/BUF634 output. Depending on how long the cable is the capacitance may range from a few tens of picofarads, to a much higher amount. Therefore, the capacitance needs to be considered relative to keeping the circuit stable. So keep that in mind.

Regards, Thomas
Precision Amplifiers Applications Engineering

Yes. The cable will be terminated so the load at the amp just looks like a 50 ohm resistor. We do a lot of super high bandwidth work so 50 ohms cables and terminations are ubiquitous to us. What I was asking was if you can suggest a lower bandwidth buffer amp. Working with high speed amps can sometimes be a pain as they can tend to oscillate. I would like the system to be fast but it just won't be. The input resistance will be 1e12 ohms or more so stray capacitance around the resistor will limit the bandwidth to a few Hertz.

So other than the BUF634, is there a lower bandwidth amp that can buffer the input stage into 50 ohms? 100-200mA output current.

Hi Mark,

Thanks for the further insight into your 200 mA application.

The OPA551 power op amp is rated for a 200 mA continuous dc output current. Its unity gain bandwidth is typically 3 MHz and is intended for lower frequency applications. Its supply range is +/- 4 V (8 V) to +/-30 V (60 V). Output voltage swing is about 3 V from the supply rails at the 200 mA output current level so it really is better suited for higher voltage supplies such as +/-15 V, and more.

You can see the OPA551 datasheet here:

www.ti.com/.../opa551.pdf

If the application uses a low voltage supply such as +2.7 V to +5.5 V, then one of TI's high current CMOS power op amps will easily deliver the 200 mA output current.

The TLV4110 can deliver up to 300 mA output current and has a unity-gain bandwidth of 2.7 MHz. Maximum output swing is typically within about 0.2 V to 0.3 V of the power supply levels at 200 mA output current.

The TLV4110 datasheet is found here:

www.ti.com/.../slos289e.pdf

For even closer swing to the supply rail capability a higher current CMOS power op amp such as the OPA567 would be a good candidate. It can deliver upwards to 2 Amps output current and has a unity-gain bandwidth of 1.2 MHz. It is packaged in a tiny 5mm x 5mm Quad Flatpack, no-lead (QFN) package. Here is its datasheet:

www.ti.com/.../opa567.pdf

Since your application presents very little in the way of a capacitive load I would not expect any stability concerns. All of these op amps are unity-gain stable and will drive up to about 1 nF without requiring external compensation measures.

Regards, Thomas
Precision Amplifiers Applications Engineering

Hi Mark,

I would do it this way:

4403.mark.TSC

Mount the BD135 and BD136 onto a heat sink.

You can reduce the supply voltages of this buffer somewhat, if you don't need the full output voltage swing.

Kai