I was just reading through past strands about active and passive filters and while they answered alot of the questions they (of course), brought up some new ones. And here it goes....
I see some filters with the RC networks woven into the feedback loops of opamps. These for me easy to identify active filters.
What is confusing me is the RC filter's with transitors after them. Without the transitor there they would be passive; does adding a tranny afterwards make it active? From what I read (from RG Keen and Rob Strand), active filters have a different drop off rate at the knee frequency. It seems that the transitor would just push the whole curve up?
I'm not 100% sure what I'm asking, but I know that I don't fully understand this and I'd really like to. So any help... would be great. :P
Thanks alot. 8)
i am probally wrong here but i always thogut the transitor after was a recovery type stage
The thing about active filters built around transistors (or op-amps), such as the ones yo'll see in any chorus, flanger, delay schematic, is that you can feed back a portion of the filter output and add resonance/emphasis around the filter rolloff. You can't do that with passive filters. In some instances that resonance/emphasis is used to add some bite or coloration to the sound. In other instances it is used as a means of mimicking a sharper/steeper rolloff with a simpler filter. For instance, the rolloffs of the 3-pole lowpass filters you'll often see built around a single transistor before and after the BBD in a delay or chorus are pretty steep for something that simply. Steepness is good because you get to hang onto more bandwidth.
If you look at some active filters you will see the opamp is connected as nothing more than a buffer (ie. - input connects to the opamp output). This type of active filter is called a (unity gain) Sallen and Key filter.
In these filters the buffer does more than just buffer, it's part of the whole feedback arrangement which lets the active filter work as it does - if you remove the buffer the filter doesn't work properly.
The filters that use transistors have the transistors wired as unity gain buffers. All that's going on here is the opamp buffer is replaced by a transistor buffer. Transistor buffers are used because they are lower cost. Because the transistor buffer isn't an ideal buffer it does change the frequency response a small amount but for the most part this can be ignored (unless you *really* want the filter to match a particular response.