# HELP!!!op-amp design problem..

#### aya

Joined Nov 30, 2006
14
using an op-amp with characteristics: open-loop gain 10000,differential input resistance 5M ohm.output resistance 0,gain-bandwidth product 65.2 MHz,an inverting and a non-inverting input.

Design a liner voltage amplifier using the op-amp and passive components to meet the specification:
voltage gain at mid-band frequency=1000
Input resistance =5K ohm
Lower 3-db frequency=100HZ
Upper 3-db frequency=2.5MHz

#### recca02

Joined Apr 2, 2007
1,214
how far have u gone with it?

#### aya

Joined Nov 30, 2006
14
well,for mid-band gain,i'm thinkin of put a resistor 9times the value of Rload to reduce the gain.
For input resistor,maybe a resistor parallel with input resistor.
Is there anything wrong with my ideas?
i dun quite get what is the purpose of the gain-bandwidth product here?

#### recca02

Joined Apr 2, 2007
1,214
my knowledge in this case is very basic
lets discuss the inverting amplifier case.
i think input resistance means the resistor u will connect as r1 in series wwith signal to the inverting i/p.
hence the feedback resistance value can be calc as
gain=feedback/input resistance.
gain bandwidth is the product of gain and freq bandwidth for which the
opamp can work.it is specified for unity gain according to my knowledge
it remains constant hence for 1000 gain it gets divided by thousand to give frequency.
since in this case we req 2.4 mhz our opamp can not have a gain of more than 65.2/2.5 so may be more than one opamp may be required. i m not sure abt this one though.
try waiting for an experienced member here,the caped crusader with a mask

#### aya

Joined Nov 30, 2006
14
Thanks. but is it necessary to use a negative feedback structure?

#### recca02

Joined Apr 2, 2007
1,214
no it is not
in case of non inverting
gain=1+feedback/input resistance
+ o/p is in same phase with input.

#### aya

Joined Nov 30, 2006
14
no it is not
in case of non inverting
gain=1+feedback/input resistance
+ o/p is in same phase with input.
sorry but i didn't make it clear,what i meant is not to use any feedback

#### recca02

Joined Apr 2, 2007
1,214
oh sorry!
then in that case how do you intend to control gain