LTSpice Beginner Question:BJT+OPAmp - Page 1
Forum section: electronics / beginners

author: pharoaz
author_rank: Newbie
timestamp: June 20, 2024, 03:43:51 pm
content: Hey, I have the following circuit with the goal of getting a 2.5 volt output and sufficient current to drive a transducer, I have the following in LTspice, however the inverting input to the op amp doesn't equal the non-inverting input of 2.5V, and with V_out = 9.77V, please advise!The results of the DC operating point analysis are below,--- Operating Point ---V(ref_2v5): 2.5 voltageV(n004): 2.5 voltageV(n002): 9.77234 voltageV(ref5v): 5 voltageV(neg_ref5v): -5 voltageV(n003): -4.11005 voltageV(n001): 5 voltageV(p001): -4.11005 voltageV(v_out): 9.77234 voltageIc(Pmbt3904): 9.11015e-12 device_currentIb(Pmbt3904): -2.29925e-11 device_currentIe(Pmbt3904): 1.38824e-11 device_currentI(C1): -1.38824e-21 device_currentI(R1): 9.61514e-10 device_currentI(R2): 9.11015e-12 device_currentI(R3): 2.29925e-11 device_currentI(R4): -1.38824e-11 device_currentI(V1): 9.61514e-10 device_currentI(V2): -0.0003 device_currentI(V3): 0.0003 device_currentIx(u1:1): -9.61551e-10 subckt_currentIx(u1:2): -1.38823e-11 subckt_currentIx(u1:99): 0.0003 subckt_currentIx(u1:50): -0.0003 subckt_currentIx(u1:34): 2.22045e-11 subckt_current

author: mawyatt
author_rank: Super Contributor
timestamp: June 20, 2024, 04:47:18 pm
content: Add a shunt to ground load resistor (100K) as the transistor has no place for its emitter current to go unless a load it attached, then see if the voltage make sense.Also what model details for the op-amp, if its behavioral then it can produce unrealistic node voltages/currents.Best,

author: magic
author_rank: Super Contributor
timestamp: June 20, 2024, 04:50:09 pm
content: Just came to say the same. Note that Ib is slightly negative, which means current flowingoutof the base - this means the base is reverse biased and the opamp tries to pull it down, but this doesn't have much effect on NPN.The unlabeled node at -4.1V is probably the output of the opamp. Hint: always look at the outputs and inputs of each individual component, try to figure out which components work as expected and which are doing something weird.

author: pharoaz
author_rank: Newbie
timestamp: June 20, 2024, 04:55:16 pm
content: yep it worked thanks a lot, appreciate it! Btw do you guys recommend any resources for analog circuit designs for beginners?

author: mawyatt
author_rank: Super Contributor
timestamp: June 20, 2024, 05:11:18 pm
content: The 1st clue that your circuit is unrealistic is the it can't produce ~9.7V when only a maximum 5V is availablePoor SPICE models, especially Behavioral, are notorious for giving bogus results, sometime blatant as here, but sometimes very subtle!!Best,

author: MathWizard
author_rank: Super Contributor
timestamp: June 27, 2024, 11:12:36 pm
content: So you want 2.5V out, what current will the transducer draw ? In these circuits there will be bias currents and offset voltages. And as someone just learning about this stuff, just those 2 things alone make a big difference to some DC circuits, when compared to ideal op-amps.There's lots of course pdf's on op-amp circuits, ideal ones and non-ideal. Some good video's on youtube too.Today I should make a proper test-jig for some op-amp DC-parameters, I just want to fit a few test circuits on a proto-board, some are easier/quicker than others.

author: Calvin
author_rank: Regular Contributor
timestamp: June 28, 2024, 04:40:52 am
content: Hi,the circuit misses on:- a gnd node as reference for LTspice ... LTSpice doesn´t like floating circuits- misses on a load (connected to gnd)- misses on a resistor from the OPAmps In+ to gnd as a dc-input-bias-pathregardsCalvin