In Reply to: transistors/op-amp amplifier posted by yong on 03/11/02 at 12:28 AM:
It all depends on the performance requirements of the amplifier. A single bipolar transistor stage can give you a maximum voltage gain of about 100 (in common emitter configuration) under ideal conditions, but as soon as you connect it to a load, this is likely to drop significantly. So, to drive the load, you might connect it to a second stage, (common collector or emitter follower); this isolates the impedance of the load from the first stage so that the gain is much less affected. If the load requires a lot of current, you might need further stages beyond the emitter follower stage. Even without loading, there is a gain limiting factor called the 'Early effect' (or base width modulation), which you can learn more about in most textbooks.
Of course, if you want a higher gain than this, you have to have another stage, or a clever technique like the current-mirror loaded differential input stage (as used in op-amps).
Another consideration that leads to the need for multi-stage amplifiers is bandwidth. The common emitter stage as mentioned above, with a gain of 100, is likely to have limited high frequency performance due to capacitances within the transistor creating unwanted negative feedback. Multi-stage connections can overcome this, delivering the required gain at higher frequencies.
Some amplifiers have to have a very high input impedance. These can be designed using a JFET or MOSFET input stage to give the high input resistance, followed by further FET or BJT amplifying stages.
Hope this gives you the general idea,