Buffer

[hykwei]

Hi Guys!

I need to construct a high speed buffer (for a 20 MHz signal)...and I don't know whether a 'NPN Common Collector Amplifier' will be suitable for this kind of situation. (The circuit that described in http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/npncc.html)

or maybe a high speed amplifier will be an easier choice? somthing like OPA633 op-amp?

Thank you for your helps!

 

[Gayan Soyza]

What are you going to switch?
Not all General Purpose Transistors suits for your job.It needs High Speed Switching transistors.Also the circuit is a common collector type.

Give us more details what you want to switch?

 

[ericgibbs]

Hi Guys!

I need to construct a high speed buffer (for a 20 MHz signal)...and I don't know whether a 'NPN Common Collector Amplifier' will be suitable for this kind of situation. (The circuit that described in http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/npncc.html)

or maybe a high speed amplifier will be an easier choice? somthing like OPA633 op-amp?

Thank you for your helps!

Do you want a HS buffer or HS amplifier [with inversion] ?
Whats the waveform shape and amplitude?
Is it a referenced to zero +/-Vsignal?

 

[hykwei]

Hi Guys

I want to build a High Speed Buffer...

I am basically building a data capture system, the receiving signal coming out from the sensor is a fixed frequency, 20MHz sinusoidal signal...amplitude varies from 20mV to 1V approximately...so I want to put a high input impedance buffer straight after the sensor stage so the signal will not be loaded or affected by the rest of the electronic circuits...

I am now thinking of PN918 transistor...but just want to know will there be some better options?

Thanks!

 

[Roff]

What is the source impedance (resistive and reactive) of your sensor? What is the impedance of the load (resistive and reactive) you want to drive? Are there any cables involved in getting the signal from the sensor to the load? The answers to all these questions are important when selecting a buffer.
Also, since the signal is a single frequency, I'm assuming you can AC couple it to the load. Is this true? if so, then the DC offset of the buffer (common collector stage) will not be an issue.

 

[hykwei]

I havn't actually measured the source impedance, but it should be fairly low...as it is simply a coil which can detect the magnetic induction signal (mutual inductance experiment...) The electronic circuits is going to have some distance away from the sensors, so it's going to be some cabling effect if I connect my sensor directly to the data acquisition system (using a co-axial cable). therefore I think putting a buffer in between the sensor and my data acqusition board is necessary....If I remember correctly the input impedance of the co-axial cable is 50 ohms)

 

[Optikon]

Hi Guys

I want to build a High Speed Buffer...

I am basically building a data capture system, the receiving signal coming out from the sensor is a fixed frequency, 20MHz sinusoidal signal...amplitude varies from 20mV to 1V approximately...so I want to put a high input impedance buffer straight after the sensor stage so the signal will not be loaded or affected by the rest of the electronic circuits...

I am now thinking of PN918 transistor...but just want to know will there be some better options?

Thanks!

Better options are high speed opamps like you mentioned. If it were me, I wouldnt go through the trouble of a multi-stage transistor amplifier design when opamp solutions are available.

 

[Roff]

Emitter followers (common collector) tend to oscillate with inductive sources and/or capacitive loads. You would be better off with a high-speed op amp, as Optikon says.

 

[ericgibbs]

Have a look at this HS opa.

 

[Bob Scott]

I think it might be the appropriate time for me to share my thoughts about using Analog Devices parts. Just pause and have second thoughts:

1. Does this part have a second source or alternative replacement?

2. Beware that when you finalize the schematic and before you send the schematic out for layout, that the pin numbers match the specified package. I know of more than one engineer with prototype boards where the multipin AD devices are hanging off the board on kynar wire legs.

Bob

 

[RadioRon]

I think it might be the appropriate time for me to share my thoughts about using Analog Devices parts. Just pause and have second thoughts:

1. Does this part have a second source or alternative replacement?

2. Beware that when you finalize the schematic and before you send the schematic out for layout, that the pin numbers match the specified package. I know of more than one engineer with prototype boards where the multipin AD devices are hanging off the board on kynar wire legs.

Bob

Hey Bob, we are fellow Vancouverites! Obviously you are a working engineer cause most of the guys on these boards dont "send the schematic out for layout" as they are usually one-man shows. I left the big company life not long ago and was happy to get back to doing my own pcb layouts for a change. Takes me back to my youth. Anyway, your post reminded me of the distaste our buyers had for Maxim parts. The parts were often unique to Maxim and the buyers had "difficulties" negotiating anything with them. So we were told to avoid. I haven't had logistic problems with AD parts, myself.