Isolating LC filters in discrete amplifiers - Page 1
Forum section: electronics / projects

author: HalFoster
author_rank: Frequent Contributor
timestamp: July 30, 2025, 12:13:57 pm
content: All-Given a couple of two transistor amplifiers - CE voltage gain, EF current gain - with a multistage LC filter in the middle: how can the filter which is highly dependent on input and output impedance be isolated from the impedances of the amplifier stages? While the theoretical impedances of the amplifier stages can be calculated, doesn't it vary with the transistors beta enough to change the filter response? Or not? It's been 35 years since I could pass a test on this and a lot of water has flowed under the bridge since then. Sigh. I could do this in a couple of minutes given the use of op-amps but I wanted to give the old school approach a try - and promptly realized just how much I had forgotten over the years. Oh, well, use it or loose it...Thanks,Hal

author: mawyatt
author_rank: Super Contributor
timestamp: July 30, 2025, 02:58:29 pm
content: Most Ladder type LC filters require a specific Load and Source Impedance to behave properly, so the transistor amplifiers need to provide the proper Impedances for the LC filter, otherwise the LC filter response will be distorted.Best

author: HalFoster
author_rank: Frequent Contributor
timestamp: July 30, 2025, 04:13:39 pm
content: Yes. Hence my question above.

author: mawyatt
author_rank: Super Contributor
timestamp: July 30, 2025, 05:01:15 pm
content: You can design the transistor amplifiers to have the proper impedances. For example assuming the CE is driving the LC filter, the Collector Resistance Rc can be selected as the proper impedance for the filter. At low frequencies this is simply Rc, at higher frequencies some of the transistor effects come into play and Rc needs to be higher than the desired filter impedance, so the combination equals the desired filter impedance.Also you can add an EF to the CE amplifier and add a series resistor which is the filter impedance desired (EF have a low output impedance approximately {kT/q)/Ie or ~26mV/Ie} for small signals). At the load end of the LC filter you can make the EF input look like the desired filter impedance by selecting the DC bias resistors to equal such, at higher frequencies the EF transistor input impedance drops and must be considered.Anyway, hope this helps.Best

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 30, 2025, 05:04:19 pm
content: Yeah in order to have more consistent gain and and input/output impedance, you will have to back off on the gain of each amplifier stage. This means terminating your amplifier inputs and outputs with resistances instead of tuned LC tanks.

author: HalFoster
author_rank: Frequent Contributor
timestamp: July 30, 2025, 05:58:53 pm
content: mtwieg & mawyatt -This is more or less what I have done so far - just looking at it and wondering at all that I was probably missing. I wasn't really worried about the input to the filter so much as it was being driven from the output of the emitter follower but i was feeling uncomfortable with my understanding of the (somewhat) complex impedance of the common emitter stage following the filter. As it is I have just plonked the filter stage, along with it's series and terminating resistances, in between two CE/EF gain stages with capacitive coupling. Multisim says happy things about the result but I just don't like a solution if I don't completely grok it. It's not a complicated application - just want to amplify a 10MHz sine (frequency standard) signal in the 30-60 mV RMS range to about 1 to 1.5 VRMS or so. As mentioned, it's something dead easy to do using op-amps but in trying to do it with discrete components I've realized that I really have a lot to relearn.Hal

author: IconicPCB
author_rank: Super Contributor
timestamp: August 02, 2025, 12:15:28 pm
content: Your other option is to provide a stage with series and shunt feedback which will offer possibility to control gain and input and output impedance of the stage. Two such single transistor stages and Bob's Your uncle.

author: mawyatt
author_rank: Super Contributor
timestamp: August 02, 2025, 07:55:40 pm
content: Quote from: IconicPCB on August 02, 2025, 12:15:28 pmThat's a good approach as the amplifier requirement has a GBW Product of >500MHz (~34dBV gain), so splitting the gain between two stages makes this easy to achieve and the 2N3904 are probably good enough!!Best

author: Tation
author_rank: Regular Contributor
timestamp: August 02, 2025, 10:38:46 pm
content: LC filters can be designed for the case that the terminal impedances are quite dissimilar. The extreme case is one of them being 0 or infinity (not sure if the case of both being 0/infinity is also feasible). In any case, an LC filter designed for one terminal impedance being 0 or infinity will perform equally well if such impedance is not 0 or infinity, but "enough" small or large.Maybe time to reach, one more time, Zverev's book...

author: mawyatt
author_rank: Super Contributor
timestamp: August 02, 2025, 11:22:13 pm
content: Quote from: Tation on August 02, 2025, 10:38:46 pmThere's a theorem called Barlet's Bisection Theorem that allows one to bisect a Ladder Type Filter which can be driven and loaded with arbitrary impedances ratios. We had a take home in grad school before the internet to prove this theorem!! We used such in the past along with Zverev's Filter bible to create filter and impedance matching networks, great resources that seem forgotten!!Best

author: IconicPCB
author_rank: Super Contributor
timestamp: August 03, 2025, 12:12:35 am
content: I got Zverev's way back in early eighties.. a weeks wage for a copy.

author: mawyatt
author_rank: Super Contributor
timestamp: August 03, 2025, 12:27:28 am
content: We had a hard copy and soft copy, lent the hard copy and it never got returned!! Still have the soft copy thoBest

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2025, 02:17:05 am
content: If the termination impedances are more critical for your operation than noise performance of those stages, a common method is to reduce the output impedance of the driver, then add a series resistance, and increase the input impedance of the stage after the filter and add a parallel resistance. Feedback can be used to reduce the output impedance and increase the input impedance at audio and RF frequencies.It's a bad design that depends directly on transistor beta, since it varies with operating point and, more importantly, varies dramatically from one 2N3904 to the next one in the DigiKey box.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 05, 2025, 06:12:27 pm
content: You were getting well paid!