thanks for the clues....here's what I did based on your input:
In[1233]:= vo = (vo /. sol3[[1]]) // Simplify
Out[1233]= (aol id Rf)/(1 + aol + C Rf s)
In[1234]:= num = Numerator[vo]/aol
Out[1234]= id Rf
In[1235]:= den = Denominator[vo]/aol
Out[1235]= (1 + aol + C Rf s)/aol
In[1243]:= den = Apart[Expand[den /. {1 + C Rf s -> 1/beta} // Simplify]]
Out[1243]= 1 + 1/(aol beta)
In[1244]:= vo = num/den
Out[1244]= (id Rf)/(1 + 1/(aol beta))
thank you.
