Hello, i need some help, i want to amplify –3dB a 1.5kHz signal with a maximum amplitude of 3V using the 2N3904 transistor.
I decided to use the common collector configuration, since I need a voltage gain of approximately 0.7 (3dB = 20log  Av ).
For the design I chose ICQ = 3.25mA, VCEQ = 10V and Vcc = 20V to have a maximum symmetric excursion.
For analysis in dc, i take IC = IE and making RE = (VccVCEQ) / ICQ calculate RE = 3k ohms.
From the data sheet, where are the graphs of the h parameters as a function of the current Ic, i take the value of hfe=140 (for ICQ = 3.25mA)
Then i calculate IB=IC/hfe= 23.21uA
VB= VBE + VRE = 0.7V + 10V = 10.7V
Assuming I2 = 10xIB > I1 = 10xIB + IB = 11xIB, with this calculate the values of R1 and R2 as
R1=(VccVB)/I1 = (VccVB)/11xIB = (20V10.7V)/(11x23.21uA) = 36k ohms.
R2= VB/I2 = VB/10xIB = 10.7V/10x23.21uA = 46k ohms.
Av = [ (RL  RE)(hfe + 1) ] / [ hie +(RL  RE)(hfe +1) ] > RL = 22 ohms.
For the capacitors i calculate C1 = 1 / ( 2*pi*1.5kHz*RIN) where RIN = R1  R2  hie
for C2 = 1 / ( 2*pi*1.5kHz*Rout) where Rout = RE  RL
But when i plot Vi and Vo with ltspice, i dont get a correct wave form for Vo, because since the voltage gain is 0.7, and taking vi as 3V, making
Av = Vo / Vi, Vo should have an amplitude of 2.1V.
Someone would be so kind to tell me what I'm doing wrong please?
I have attached the ltspice files in case it works for you.
I decided to use the common collector configuration, since I need a voltage gain of approximately 0.7 (3dB = 20log  Av ).
For the design I chose ICQ = 3.25mA, VCEQ = 10V and Vcc = 20V to have a maximum symmetric excursion.
For analysis in dc, i take IC = IE and making RE = (VccVCEQ) / ICQ calculate RE = 3k ohms.
From the data sheet, where are the graphs of the h parameters as a function of the current Ic, i take the value of hfe=140 (for ICQ = 3.25mA)
Then i calculate IB=IC/hfe= 23.21uA
VB= VBE + VRE = 0.7V + 10V = 10.7V
Assuming I2 = 10xIB > I1 = 10xIB + IB = 11xIB, with this calculate the values of R1 and R2 as
R1=(VccVB)/I1 = (VccVB)/11xIB = (20V10.7V)/(11x23.21uA) = 36k ohms.
R2= VB/I2 = VB/10xIB = 10.7V/10x23.21uA = 46k ohms.
Av = [ (RL  RE)(hfe + 1) ] / [ hie +(RL  RE)(hfe +1) ] > RL = 22 ohms.
For the capacitors i calculate C1 = 1 / ( 2*pi*1.5kHz*RIN) where RIN = R1  R2  hie
for C2 = 1 / ( 2*pi*1.5kHz*Rout) where Rout = RE  RL
But when i plot Vi and Vo with ltspice, i dont get a correct wave form for Vo, because since the voltage gain is 0.7, and taking vi as 3V, making
Av = Vo / Vi, Vo should have an amplitude of 2.1V.
Someone would be so kind to tell me what I'm doing wrong please?
I have attached the ltspice files in case it works for you.
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