Lets take a look on a linear amplifier - Page 1
Forum section: electronics / projects

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: September 11, 2019, 08:30:05 am
content: Hi there,Hope my post find you well.I like to get attention the knowledgeble person in this post !So, kindly avoid this post if you wont have experience.I wanna post a circuit that may be explained by Dr. Kolar, a famous Swiss or Austrian proffesor.I need to know this application well, because I want to develope a 3-phase-calibrator.My application needs to amplify signal to a certain level. Class D switching is necessary!Here we go ....Explain kindly1. Why we need to think SNR for this case?2. Why we need to think frequency gain voltage booster? Loop gain!3. Explain impedences, transcondactance etc for whole circuit.4. How the voltage and current is being controlled here?Thanks a lot!

author: moffy
author_rank: Super Contributor
timestamp: September 11, 2019, 09:17:02 am
content: This is just a very basic MOSFET output linear amplifier. No class D switching here. The tricky part is biasing the output FETs over temp to avoid:1. Too much cross over distortion2. Too much idle current.You need to explain your end application better if you want help.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: September 11, 2019, 01:06:07 pm
content: Dear Sir Moffy,You could start with some information that is offcourse necessary at this moment.In case of frequency domain, we cant see anything in feedback path of amplifier. Signal amplification in both side is slightly understandable.I am thinking to use TIP142 , C1815, A1050, TIP147 for both current and voltage amplification. Like darlington pair, one transistors collector can be connected to others base.As you said distortion matters.My application generates a lot of heat, because it 5 A application.

author: Benta
author_rank: Super Contributor
timestamp: September 11, 2019, 07:17:39 pm
content: Your schematic is a fairly classic symmetric linear audio power amplifier with MOSFET output stage. Most probably designed to run in Class B or AB mode.I'd say, based on the supply voltages, that it's designed for around 200 W output power.Nothing Class D about it, but you can of course feed it with a square wave signal if you like.Power stage quiescent current is set with the "~10 V" voltage in the middle (which should be thermally coupled/compensated with the output MOSFETs).I have no idea why you want to use such "whopper" transistors as the TIP142/147. All the MPSAxx types are small-signal and uncritical, The BF871/872 MOSFET drivers are CRT horizontal transistors and are high voltage and high frequency, which is needed in this circuit.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: September 12, 2019, 03:49:25 am
content: Thank you to take a part here.My current design might be class AB then!!Its feedback opamp is 551, input series RC element is 13k, 1uF.Feedback parallel element may be 4.7 k, .33uF. Output has .33u and 10ohm series RC.I am using TIP147, TIP142, C1815, A1015. They are properly biasd like cascaed 3 stage, may be AB class network. More symmetric way, like one pairs AB class output is connected to others pair. Last pairs base is connected to first pairs base! At last a voltage divider of 5 ohm has been made between 2nd and 3rd outputs!Biasing voltage of this amplifier is plus, minus 24 volt.The problem I am facing is heating !

author: moffy
author_rank: Super Contributor
timestamp: September 13, 2019, 12:37:34 am
content: The feedback is the resistor from the output of the amp to the -ve input of the opamp. This with the input resistor sets the amp gain. The series RC network around the opamp is for stability purposes. You don't need the OPA551 as the opamp, it doesn't need much of a supply voltage as the output only has to drive +/-10v . If you put clamping diodes on the -ve input you will prevent the output injecting overvoltage. Stability is a real issue. You need small ferrites between the gate of the FETs and the driver to prevent VHF oscillations of the output stage.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: September 13, 2019, 05:12:40 am
content: Quote from: moffy on September 13, 2019, 12:37:34 amNice explanation indeed.In my case, bothside biasing is being well protected for DC smoothing. Feedback op-amp works at + and -24 volt. At 551 output, parallel combinaton of 2 ohm and 2k has been made for voltage divider, that was fed to C1815 and A1015 to complete 3 stage AB class.Two 1N4148 is also connected oppositly on 551 output thus bothside signal can easily conduct.Actually I have a control board, made from STM32.15 VAC and 8 VAC stepdown transformer has fed to the system, may be regulated ADC/ DAC has been converted, scalled to a level. So, if you press decimal numbers on the key pad, certain signal will be produce for linear amp input.Since its a power application, I was testing my CT and PTs, those are connected to this linear Amp output. In this case a power transformer ( having center taps for diffirent volatge ) is also used!CT gives maximum 6A( if you press larger number on keypad) while PT gives maximum 360 Vac.At this time TIP147, TIP142 shows steady state.It succeed the aging test. But, at higher level of feedback amplifiers input, it shows high heat!I think, there is swithing loss, heat loss. For the seek of high efficiency, I need to think Class D.Still this design makes money, but need to me more advanced and smart on duty.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2019, 07:22:00 am
content: The voltage boost stage runs with a lot of current. This makes it run hot, very close the absolute limits for the small transistors.In addition the doubled long tail pair can cause drift of the standing current. I may be OK with the rather large emitter transistors, but normally it is not such a good idea. So this looks more like a 1 st crude idea and not a practical circuit.Using TIP142/147 makes it a different amplifier. The C1815 are much lower voltage.With modern BJTs there is no much need to use MOSFETs in linear mode.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: September 13, 2019, 12:01:22 pm
content: Quote from: Kleinstein on September 13, 2019, 07:22:00 amYou are talking very close to the point I need.I think if you can show a related circuit then I could understand easily. Voltage booster and current sink capabilities are very important here. As you have meintioned " standing current". " A long tail design" is crying need at this moment.In my design, 3 stages are used in voltage booster or current pumping. Due to full range of signal, upper side contains C1815, then TIP142 and TIP142.At lower portion, A1015, TIP147, TIP147 complets 3 stage in order.There must be a reason why at the 1st stage normal BJT(NPN,PNP) is used.Class D needs PWM must.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2019, 12:30:07 pm
content: The voltage boost part is needed because the OP can not directly drive the +-85 V swing to derive the MOSFETs.At lower supply voltage an OP my be suitable. Using C1815 and similar transistors suggests that the different circuit uses a much lower supply.However the voltage boost stage as shown uses way to much current for the small transistors.With Darlington transistors instead of the FETs the bias circuit should also be different. So chances are one would start all over and forget about the circuit shown.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: September 16, 2019, 01:40:25 am
content: Quote from: Kleinstein on September 13, 2019, 12:30:07 pmYou are helping me a lot!Yeap, for this reason C1815 and A1050 is used.Quote.Then what other ways we can get a good result here?QuoteI will be happy if you can post a general design what you have said.Now lets come to the point, I am talking about the feedback amplifier1. My voltage and current amp circuit is slightly different.2. In voltage amp, at OPA551, input signal has feed with reference to "ground". A series RC element like 1k, 1uF after that a 220k (one end grounded) are connected to " non- inverting" input. Also a 220k one end ground is conneted to"inverting" input. A parallel RC element 270k, 0.33uF(may be) is connected to feedback path. A10 ohm, 0.1 uF RC element in series also appears in the output of the amp! Output signal also has ground reference.3. In current amp, at OPA551, input signal is feed with reference to " ground". A series RC element like 13k, 1uF after that a 1k ( one end grounded) are connected to " non inverting" input. Also a 1k single ended gound is also conneted to "inverting" input. A parallel RC element (4.7k and 47uF) is connected to feedback path. A 10 ohm, 0.1 uF RC element in series are also appears between the output of the amp! In this case 2 shunt element 5W 0.5 ohm are connected and grounded in output side. No reference ground output here!