# Multistage BJT Amp

#### wtrow

Joined Oct 21, 2009
18
So I need to design a multistage amp using bjt's (npn and/or pnp). I need a midband gain of 200, F_low of 100 Hz, and F_high of 22 kHz. I have tried some CE-CE and cascade configurations, and I've been getting close to the right gain, but my bandwidth is way too large (1-10MHz). I've been searching for ways to narrow the bandwidth but I can only find ways to make it wider.

Anyway, what I'm trying to ask is does anybody know a configuration of bjt amps that could get me a narrow bandwidth?

#### jegues

Joined Sep 13, 2010
733
So I need to design a multistage amp using bjt's (npn and/or pnp). I need a midband gain of 200, F_low of 100 Hz, and F_high of 22 kHz. I have tried some CE-CE and cascade configurations, and I've been getting close to the right gain, but my bandwidth is way too large (1-10MHz). I've been searching for ways to narrow the bandwidth but I can only find ways to make it wider.

Anyway, what I'm trying to ask is does anybody know a configuration of bjt amps that could get me a narrow bandwidth?
You could create a band-pass filter somewhere within the amplifier to achieve the desired bandwidth.

#### Audioguru

Joined Dec 20, 2007
11,249
Simple.
A simple CR highpass filter cuts low frequencies.
A simple RC lowpass filter cuts high frequencies.

Post your schematic showing resistor and capacitor values.

#### wtrow

Joined Oct 21, 2009
18
Simple.
A simple CR highpass filter cuts low frequencies.
A simple RC lowpass filter cuts high frequencies.

Post your schematic showing resistor and capacitor values.

I'll try that and post my cadence schematics in a little bit. Thanks for the idea. However, I'm not sure if my professor 'wants' us to use RC filters. He has been really pushing using multiple BJT amps. I haven't had the chance to ask him yet, but I will do. If there is a way to narrow bandwidth with BJT's I would love to know, otherwise I'll stick with the RC filter.

#### Audioguru

Joined Dec 20, 2007
11,249
Germanium transistors from the 50's and 60's had poor high frequency response.
The transistors have not been made for many years but some people hoarded them.
I'll sell you a couple of AC128 germanium transistors for $100.00 each. A modern old 2N3055 silicon power transistor has poor high frequency response when its gain is high. Try some. Any common-emitter transistor has poor high frequency response when a capacitor is added from collector to base and the source impedance is fairly high. Thread Starter #### wtrow Joined Oct 21, 2009 18 Germanium transistors from the 50's and 60's had poor high frequency response. The transistors have not been made for many years but some people hoarded them. I'll sell you a couple of AC128 germanium transistors for$100.00 each.

A modern old 2N3055 silicon power transistor has poor high frequency response when its gain is high. Try some.

Any common-emitter transistor has poor high frequency response when a capacitor is added from collector to base and the source impedance is fairly high.

I'm actually going to be making this in a class, where we will be supplied 2n3904 and 2n3906 BJT's, so I won't need to get any. Anyway, I ran some simulations in cadence. I tried the RC filter, and tried using all sorts of values, yet I couldn't get my band-pass low enough.

The load on the end with vout is the oscilloscope probe.

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#### jegues

Joined Sep 13, 2010
733
I'm actually going to be making this in a class, where we will be supplied 2n3904 and 2n3906 BJT's, so I won't need to get any. Anyway, I ran some simulations in cadence. I tried the RC filter, and tried using all sorts of values, yet I couldn't get my band-pass low enough.

The load on the end with vout is the oscilloscope probe.
We can select the points at which we want the our bode plot to flatten out and then drop again by selecting RC combinations correctly.

The 2 points will be dictated by, $$\frac{1}{\tau} = \frac{1}{RC}$$

Where R is the resistance as seen by the capacitor.

#### Audioguru

Joined Dec 20, 2007
11,249
Your simulator shows such a low output level that its frequency response curve is useless.

I guess you have not been taught the simple formula for the cutoff frequency of a resistor and capacitor.

I converted your negative schematic to a normal positive one with a white background.
I figure that the bandwidth is from about 1Hz to about 492kHz.

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#### wtrow

Joined Oct 21, 2009
18
I came up with a different design and it works now. Thanks for the help.

#### Audioguru

Joined Dec 20, 2007
11,249
Thanks for showing everybody what you changed.